404 



NATURE 



[August 25, 1887 



hitherto been considering this principle — or with reference to use 

 and disuse— we have here a consideration of great importance in 

 regard to the subject of Prof. Lankester's essay above quoted. 

 Apparently without having either heard or thought of the prin- 

 ciple of cessation, Dr. Dohm was led to attribute an important 

 part in the drama of evolution to the effects of cessation, as these 

 are witnessed in the phenomena of degeneration.^ About the 

 facts of degeneration there can be no doubt, and to this naturalist 

 belongs the credit of having first perceived the wide range of 

 their importance. But, on account of having missed the principle 

 of cessation, both Dr. Dohm and his English expositor, Prof. Lan- 

 kester,^ fell into an omission of interpretation. For they both at- 

 tributed the facts of degeneration to a reversal of natural selection ; 

 they represented that degeneration could only take place under 

 a change in the conditions of life such that organs or structures 

 previously useful become, not merely useless, but deleterious. 

 Degeneration was thus regarded as always the result of what may 

 be termed active hostility on the part of natural selection ; not 

 as the result of a merely passive disregard. Hence the sphere 

 within which the phenomena of degeneration might be expected 

 —or admitted of being satisfactorily explained — was needlessly 

 limited. For instance. Prof. Lankester writes : " It is clearly 

 enough possible for a set of forces such as we sum up in the 

 term ' natural selection ' to so act on the structure of an organism 



as to diminish the complexity of its structure." But in 



order "to diminish the complexity" of any useless structure, it 

 is not neces-ary that natural selection should "act on the 

 structure " : the complexity, like the size, of the structure would 

 necessarily diminish under the mere withdrawal of selection. 

 And hence the phenomena of degeneration do not require, either 

 that the organism presenting them should ever have found its 

 useless organs actively deleterious, or that there should ever have 

 been any " Functions- wechsels " in the case.^ 



The case of degenerated complexity proves that the cessation 

 of selection may effect degradation without assistance from the 

 economy of nutrition. I am therefore more disposed to think 

 that the size of any useless structure may be reduced to a greater 

 extent by the mere cessation of selection (apart from economy), 

 individual cases fall below 25 per cent., and of these two should be omitted 

 ill ' ^^"^''°"'' P- 272). Thus, out of eighty-three examples, only two 

 fall below the lowest average expected {i.e. on the supposition that disuse 

 has not had anything to do with the reduction). Moreover, we should 

 scarcely expect disuse alone to affect in so similar a degree such widely 

 different tissues as are brain and muscle. The deformity of the sternum in 

 fowls also points to the cessation of selection rather than to disuse. Further, 

 the fact that several of our domestic animals have not varied at all is inex- 

 plicable upon the one supposition, while it affords no difficulty to the other. 

 We have seen that disuse can only act by causing variations ; and so we 

 can see no reason why, if it acts upon a duck, it should not also act upon a 

 goose. But the cessation of selection depends upon variations being supplied 

 to It ; and so, if from any reason a specific type does not vary, this principle 

 cannot act. Why one type should vary, and another not, is a distinct ques- 

 tion, the difficulty of which is embodied by the one supposition, and excluded 

 by the other. For, to say that disuse has not acted upon type A, because of 

 Its inflexible constitution, while it has acted on a closely allied type B, 

 because of its flexible constitution, is merely to insinuate that disuse, having 

 proved itself inadequate to cause reduction in the one case, may net have 

 been the efficient cause of reduction in the other. But the counter-supposi- 

 tion altogether excludes the idea of a causal connection, and so rests upon 

 the more ultimate fact of differential variability, as not requiring to be ex- 

 plained. Lastly, it is remarkable that those animals which have not suffered 

 reduction in any part of their bodies are likewise the animals which have not 

 varied m any other way, and conversely ; for as there can be no causal con- 

 nection between these two peculiarities, the fact of the intimate association 

 between them tends to show that special reduction depends upon general 

 variability, rather than that special variability depends upon special reducing 



' ," Der Ursprung der Wirbelthiere und der Princip des Functions- 

 ■wechsels (Leipzig, 1875). 



\ ^P^^*^"^"""" ■ ^ Chapter in Darwinism " (London, i88o). 



^ The same considerations apply to the size of an organism as a whole. If 

 ^ny reason it ceases to be an advantage to be kept up to the ancestral 

 standard of size, the cessation of selection as regards size would result in a 

 gradual diminution of size, even though the ancestral standard of size were 

 not actually deleterious. Yet, in the last of the passages above quoted from 

 i'rof. i-ankester— and the passage in his essay where he most nearly 

 approaches the principle of selection as withdrawn— the context shows that 

 he only has in view the principle of selection as reversed. For he says :— " It 

 cannot be doubted that natural .selection has frequently acted on a race of 

 animals so as to reduce the size of the individuals. The smallness of size has 

 been favourable to their survival in the struggle for existence." Of course 



it cannot be doubted " that this has been so in many cases ; but as little 

 can It be doubted that it has not been so in all. In any given case of diminu- 

 tion, it IS not necessary to suppose that "the smallness of size has been 

 favourable m the struggle for existence" : it is enough if the previous large- 

 ness of size has ceased to be so, or that smallness of size is no longer delete- 

 rious. Moreover, the same considerations apply to instincts. For example, 

 It can scarcely ever have been a. fatal disadvantage to the slave-making ants 

 thatthey should be able to eat their own food ; therefore the loss of their 

 original instincts, which now renders them dependent on their slaves for being 

 led, can only have been brought about by the cessation of selection— not by 

 Its reversal. 



than I thought when writing the articles above quoted. Here, 

 however, we must remember that the hold which heredity has 

 upon complexity is much less than that which it has upon size. 

 This is evident, not only from obvious considerations of an a 

 priori kind, but also from such cases as those of the blind 

 crabs of Kentucky. Here the disused eyes have been lost, 

 while the foot-stalks which originally supported them have 

 been retained. Now, we can well understand why the eyes 

 should have been the first to disappear under the cessation of 

 selection, seeing that they were structures so highly organised 

 that the continuous influence of selection must have been required 

 to preserve them in a state of efficiency before the animals began 

 to inhabit the dark caves ; and, therefore, that when the animals 

 did begin to inhabit these caves, such refined and complex 

 structures would rapidly degenerate through the mere withdrawal 

 of selection. But if we were to attribute any large share in this 

 process of rapid degeneration to the economy of nutrition, we 

 .should be unable to explain the persistence of the foot-stalks. 

 Therefore, the cessation of selection, when acting alone, is thus 

 proved capable of reducing a complex structure more quickly than 

 it can reduce a larger but less complex structure, in the same 

 species and under the same conditions. 



It is true that in a passage above quoted, and which was 

 published two years before Dr. Dohrn's essay, I myself attributed 

 the phenomena of degeneration to a "reversal of natural selec- 

 tion." 1 But I alluded to such reversal only in so far as it arose 

 from the economy of nutrition {i.e. I did not suppose that de- 

 generation can only occur when useless parts become actively 

 deleterious, and therefore require the active agency of selection 

 to remove them) ; and the effect of reading the subsequently 

 published literature on the subject of degeneration has been to 

 make me attribute more importance to the cessation of selection, 

 and less importance to the economy of nutrition. Nevertheless, 

 I still believe that these principles are inadequate to explain the 

 final and total obliteration of organs which by their combined 

 action they have rendered rudimentary. 



And these remarks lead me to indicate the points wherein my 

 hypothesis of the cessation of selection differs from that which 

 has recently been published by Prof. Weismann. Briefly, he 

 does not mention the assistance which this principle derives from 

 that of the economy of nutrition, and he believes that it is in 

 itself sufficient to explain the final and total obliteration of useless 

 parts. Having already given my reasons for holding different 

 views with regard to both these points, it will now suffice merely 

 to re-state the principles which I suggested in the Nature 

 articles as having been most probably concerned in this final and 

 total obliteration of useless parts. These principles are two in 

 number, and are both quite independent of those which we have 

 hitherto been considering. The first of them is inheritance at 

 earlier periods of life, which progressively pushes back the 

 development of a useless rudiment to a more and more embryonic 

 stage of growth ; and the second is the eventual failure of the 

 principle of inheritance itself. For, " whether or not we believe 

 in Pangenesis, we cannot but deem it in the highest degree im- 

 probable that the influence of heredity is of unlimited duration. "- 

 This view of the matter renders it abundantly intelligible why 

 it is that, when once the cessation of selection — co-operating with 

 the economy of nutrition — has with comparative rapidity reduced 

 any useless organ to a rudiment, the latter should then persist 

 for so enormous a length of time that in the result, as Mr. Darwin 

 observes, " rudimentary organs are so extremely common that 

 scarcely one species can be named which is wholly free from a 

 blemish of this nature." 



We have seen that in the cessation of selection we must 

 recognise one of the principal causes of atrophy in species ; 

 in whatever measure we hold the presence of selection ex- 

 planatory of evolution, in a corresponding measure must we 

 hold the withdrawal of selection accountable for degeneration. 

 But from this it does not necessarily follow that no other causes 

 either of evolution or of degeneration are to be found. Those 

 naturalists who adopt the light and easy method of out-Darwining 

 Darwin, or close their eyes to every other "factor" save that of 

 natural selection, may indeed rest satisfied with these two com- 

 plementary principles as in themselves adequate to explain all 



1 Prof. Weismann christens the principle which I have called Cessation of 

 Selection, Keltrseite der Naturziichtitng ; but, for reasons above given, I do 

 not think that this is so good a name as that which he elsewhere uses 

 incidentally, and which, indeed, is an unconscious translation of my own term 

 — namely, Nachlass der Natjirziichtung. 



^ Nature, toe. cit., where see for a fuller discussion of the causes leading 

 to eventual and total suppressitn. 



