294 



NATURE 



[July 30, 1896 



I must add a few words about specific dificrences. Some one 

 has recently observed that Mr. Darwin has sjiven no definition 

 of a species. I do not propose to attempt the task. But the 

 majority of botanists demand that a "good species" shall be 

 distinctly marked oft" from every other by definite and tangible 

 characters. The members of the group may either conform 

 pretty closely to a common type, or exhibit a good deal of 

 variation amongst themselves, and this variation may be some- 

 times indifferent, soinetimes adaptive. Such a variable group 

 is generally considered to contain species " on the make " ; but 

 the indifferent variation will be remorselessly, if slowly, brought 

 to book by natural selection. A botanical species is then a 

 discontinuous group marked off by characters which I believe to 

 be adaptive. 



I have a strong suspicion that zoologists have a different con- 

 ception of a species from that of botanists. I once heard Prof. 

 IIu.Kley say roundly, at a meeting of the IJnnean Society, that 

 there were no such things as species at all. The subject under 

 discussion was a group of Salmonidic, and he said that if the 

 forms were arranged in a row, it was a jiurely arbitrary matter 

 how any one chose to cut it up into species. But the same 

 state of things might be paralleled amongst plants — as, for 

 example, in Hicraciiim. The occurrence of such cases is not 

 incompatible with the fact that the majority of species probably 

 admit of being sharply defined, and are, in other words, discon- 

 tinuous. This is, at any rate, the case with plants, and I do 

 not see why it should not be equally so with animals. If, how- 

 ever, zoologists cut their species finer, it is intelligible that they 

 may find difficulty in recognising the distinctive characters as 

 adaptive. Prof. Poulton, in the discussion at the Linnean 

 Society, went the length of saying that he saw no objection to 

 giving a name to every distinct form, leaving it to be afterwards 

 decided if it were or were not entitled to specific rank. Such a 

 proceeding, if general, would throw taxonomy into a state of 

 chaos. It has been adopted by a few botanists ; but by 

 common consent their writings, though not without a certain 

 interest as studies in variation, have been excluded from serious 

 taxonomic literature. W. T. Thiselton-Dyer. 



Kew, July 20. 



In his letter, published in Nature of July 16, Prof. 

 Lankester has formulated with great clearness his views con- 

 cerning the utility of specific characters ; and he explains that 

 his chief object in doing so is to draw attention to certain state- 

 ments of mine, which he declares to involve a serious logical 

 fallacy. While I am grateful for the courtesy with which Prof. 

 Lankester has tempered his condemnation of my logic, I 

 am still unconvinced ; and the point at issue is so important 

 that I am anxious to state, as clearly as I can, what my own 

 position is. I may perhaps conveniently begin by quoting in 

 full a passage from a former paper. Last year I wrote as 

 follows ; 



" In order to estimate the effect of small variations upon the 

 chance of survival, in a given species, it is necessary to measure 

 first, the percentage of young animals exhibiting this variation ; 

 secondly, the percentage of adults in which it is present. If 

 the percentage of adults exhibiting the variation is less than the 

 percentage of young, then a certain percentage of young 

 animals has either lost the character during growth, or has been 

 destroyed. The law of growth having been ascertained, the 

 rate of destruction may be measured ; and in this way an 

 estimate of the advantage or disadvantage of a variation may 

 be obtained" {Roy. Soi. Proc, vol. Ivii. p. 381). 



Prof. Lankester objects to this passage ; and, if I understand 

 him rightly, his objection may be stated in this way : — Admitting 

 it to be proved that variation in a certain dimension, among 

 young animals of a sjjecies, is associated with change in the 

 death-rate, so that when this dimension increases the death-rate 

 increases, and when it diminishes the death-rate diminishes ; so 

 that by ascertaining the magnitude of this dimension in a young 

 animal you can accurately measure its chance of becoming adult ; 

 — admitting this relation to hold through a range of experience 

 sufficient to form the basis of a reasonaljle induction, you have 

 .still no right to say that change in the observed dimension is a 

 cause of the subsequent change in death-rate; for the two 

 observed phenomena, namely the change in the observed 

 dimension and the subsequent change in the death-rate, may 

 alike be due to variation in some unobserved character, which 

 alone is effective in causing change of death-rate. 



NO. 1396, VOL. 54] 



In other words, you have a phenomenon, namely death-rate, 

 preceded invariably by two or more phenomena of structure or 

 function ; and these are so associated, that from a known change 

 in the antecedent group of phenomena, affecting always every 

 member of tlie group, you can infer a change of known magni- 

 tude in the death-rate. Under these circumstances. Prof. 

 Lankester thinks it legitimate to pick out one of these antecedent 

 phenomena, and to speak of it as the only effective cause of 

 change in death-rale, the other changes, although equally 

 universal, being merely unimportant concomitants of this one 

 essential change. He further finds .something extraordinary in 

 my logical position when I disagree with him, and considers 

 every member of the group of correlated changes which in- 

 variably |)recedes change in death-rate as one of the causes of 

 that change. 



I have ventured to restate Prof. Lankester's position in my 

 own words, in order to show what I believe him to mean. If I 

 have in any way misrepresented him, I trust he will forgive me. 



My own view seems to me identical with tliat held by a large 

 number of persons, from Hume onwards ; and for that reason I 

 hope Prof. Lankester will not think I am indulging in an "empty- 

 wrangle" if I ask whether he accepts the following statement : 



" We may define a cause to be an object, followed />y another, 

 and where all the objects, similar to the Jirst, are followed by 

 objects similar to the second, or in other words, where, if the first 

 object had not been, the second neiier had existed. . . . We may 

 . . . suitably to experience, form another definition of cause, 

 and call it, an object, followed by another, and whose appearance 

 always conveys the thought to that other. But though both these 

 definitions be drawn from circumstances foreign to the cause, we 

 cannot remedy this inconvenience, nor attain any more perfect 

 definition, which may point out that circumstance in the cause, 

 which gives it a connection with the effect. We have no idea 

 of this connection ; nor even any distinct notion of what 

 it is we desire to know, when we endeavour at a conception 

 of it" (Hume: "Inquiry concerning Human Understanding," 

 §vii.). 



When I have spoken of cause and effect, I have always 

 endeavoured to use the words in accordance with the definition 

 given in this passage or in Kant's extension of it ; but Prof 

 Lankester seems to go beyond it. At least, the process of 

 selecting one out of a group of universal antecedents, and 

 calling that one alone the effective cause of the consequent, 

 seems to me to involve precisely that knowledge which 

 Hume and all his followers disclaim. For unless he knows 

 " that circumstance in the cause which gives it a connection 

 with the effect," how does Prof. Lankester pick out that 

 one of the universal antecedents of an event which he chooses 

 to call the cause? Such selection w-ould have been im- 

 possible to Hume ; and if Mill had regarded it as possible, he- 

 would hardly have defined a. cause as " the sum total of the con- 

 ditions, positive and negative taken together ; the whole of the 

 contingencies of every description, which being realised, the 

 consequent immediately follows." 



It is the assumption of the right to choose one out of a number 

 of universal antecedents, and to regard this as the only cause of 

 the consequent, which I have ventured to call illogical : and 

 since Prof. Lankester has quoted Mill against me, I would ask 

 him to read Mill's opinion of such a proceeding. 



The prevalence of this practice in biological speculation tends 

 more than any other habit to that neglect of the real complexity 

 of the phenomena of life which Prof. Lankester himself so justly 

 deprecates. For example, the contraction of the body of an amieba 

 has been discussed of late years in two ways. Observers, follow- 

 ing Prof. Lankester's method, have discussed the question, how 

 much of the body of an amoeba is the effective cause of its con- 

 tractility? It is possible roughly to divide the body of an 

 amoeba (neglecting the nucleus) into an ap|xirently more solid 

 net-work or sponge-work, and an apparently more fluid 

 substance in the meshes of this sponge-work. The question has 

 been hotly debated, which of these two substances should be 

 regarded as the essentially contractile element, the other being 

 an unimportant concomitant. Each alternative has had its 

 advocates, and neither party has convinced the other. Readers 

 of N.\TURF, are aware that a short time ago Prof. Biitschli 

 attacked this question from the standpoint which I am here 

 advocating : that is to say, he regarded each of the substances, 

 invariably antecedent to contraction, as one of the causes of the 

 contraction. By considering the changes in the relation between 

 the two. Prof. Biitschli was at least enabled to make a dead 



