August 14, 1890] 



NATURE 



369 



The subject with which the author deals is the relation of 

 Japan to the Eastern Question, and therefore to England 

 and Russia. It is a striking fact that such matters should 

 be discussed in an English work by a native of Japan. 

 Mr. Manjiro Inagaki cannot, however, be congratulated 

 on the way in which he sets forth his ideas. His facts are 

 thrown together so loosely that it is sometimes difficult to 

 make out the propositions which he wishes to prove or to 

 illustrate. 



LETTERS TO THE EDITOR. 



[ The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications. ] 



Indiscriminate Separation, under the Same Environ- 

 ment, a Cause of Divergence. 



I HAVE accumulated a large body of facts indicating that 

 separated fragments of a species, though exposed to the same 

 environment, will in time become divergent. I find that, 

 wherever a species possessing very low powers of migration is 

 for many generations divided into a series of fragments by 

 barriers that do not obstruct the distribution of surrounding 

 species, more or less divergence arises in the separated portions 

 of the species, though, in the same areas, there is no divergence 

 in the environing species whose distribution is not obstructed. 

 I still further find that, whenever the distances intervening 

 between the different fragments are an approximate measure of 

 the time and degree of separate breeding (as is frequently the 

 case, as long as the divergence does not involve any physio- 

 logical and psychological segregation), these distances are also 

 an approximate measure of the degree of divergence. 



The validity of this conclusion is called in question because it 

 is inconsistent with the theory that all divergence is due to 

 diversity of natural selection, and that all diversity of natural 

 selection is due to exposure to different environments. The 

 divergences in the cases above referred to, it is said, are 

 probably due to differences in the environment that are not 

 easily recognized. This was the explanation suggested by 

 Darwin when the facts were reported to him in 1872. The 

 division of a species into isolated portions did not seem to him 

 to furnish any factor that could produce divergence unless it was 

 aided by exposure to different external conditions. The same 

 view is expressed in his " Origin of Species," sixth edition, 

 P- 319- 



My reply is twofold, (i) The theory that all the divergences 

 in Sandwich Island land mollusks are due to differences in the 

 environment requires us to believe that there are occult in- 

 fluences increasing in difference with each additional mile of 

 separation, and that these influences control the natural selection 

 of the mollusks, but have no influence on any of the other 

 species occupying the same areas. A theory that involves so 

 heavy an assumption cannot be received when a simpler theory 

 is open to us, (2) I believe I can entirely remove this objection, 

 urged against my conclusion on these purely theoretical grounds, 

 by showing that there are certain causes of divergence, not de- 

 pending on exposure to different environments, that are neces- 

 sarily introduced by the division of a species into isolated 

 groups ; and that, under the influence of these causes, diversity 

 of habits may arise producing diversity of natural selection, even 

 while the fragments are exposed to the same environment. 



I have elsewhere called attention to the fact that the inde- 

 pendent breeding of separated groups, so far as we can judge, 

 always tends to produce divergence; and I have shown that, 

 when a species is indiscriminately broken into independent 

 fragments, the tendency to divergence will, on the average, vary 

 in direct proportion to the instability of the species, and in 

 inverse proportion to the size of the fragments, for on these 

 factors depends the probable degree of departure of the average 

 characters of the fragment from the average character of the 

 species previous to its being broken into fragments, and, 

 therefore, the degree of segregation. 



I wish now to show that the maintenance of certain classes of 

 characters always belonging to an unbroken species is due to a 

 form of selection that can continue only so long, and so far, as 



free crossing continues. Reflex selection is a formative prin- 

 ciple, depending on the relations in which the members of an 

 inter-generating group of organisms stand to each other, while 

 they continue to inter-generate ; but when two portions of an 

 original species have become so divergent as to compete with 

 each other in the same area without crossing, they form in- 

 cipient species, and each belongs to the environment of the 

 other. While they are members of the same inter- generating 

 group, their mutual influence results in reflex selection, which 

 maintains the correspondence with each other by which power 

 to cross is preserved ; while they are members of groups that do 

 not cross, their mutual influence results in mutual selection that 

 inevitably tends toward the preservation of variations that, 

 through greater divergence, best escape from competition. I 

 have elsewhere defined reflex selection as being the exclusive 

 propagation of those better fitted to the relations in which the 

 members of the same species stand to each other, resulting from 

 the failure to propagate of those less fitted. Among those that 

 are equally fitted to the environment of the species, and there- 

 fore equally preserved by natural selection, there is often great 

 difference in the degrees of fitness for sustaining such relations 

 to the rest of the species as will secure an opportunity to pro- 

 pagate. To this class of influences belong the different forms of 

 sexual selection through which the sexual instincts and other 

 sexual characters of the different sexes are kept in full co- 

 ordination. In like manner we must beheve that the pollen of 

 any species is kept up to its full degree of potency by the con- 

 stant selection which results from the failure to propagate of the 

 individuals whose pollen is less potent or whose germs are more 

 difficult to fertilize than the average. We cannot call this 

 sexual selection ; but we have to class it as the form of reflex 

 selection through which the physiological co-ordination of the 

 male and female elements with each other is so maintained as to 

 secure full fertility. Again, there is a constant selection of 

 animals that are suitably endowed with the recognition marks 

 and calls by which the different members of the species know 

 each other, and that have the corresponding instincts that lead 

 them to associate with their own kind who are thus endowed. 

 I have elsewhere called this principle of social co-ordination 

 "social selection," and have classed it as a form of reflex 

 selection. 



There are several other forms of reflex selection. One of 

 these secures harmony in the habits and modes of life of the 

 different members of a freely inter-generating group of organisms ; 

 for individuals, whose habits are not sufficiently co-ordinated 

 with those of the mass of the species to allow of their inter- 

 breeding with them, will fail of propagating. This we may call 

 co-ordinative industrial selection. We are now prepared to 

 understand one reason why independent breeding resulting from 

 indiscriminate separation is in time transformed into segregation. 

 Independent breeding is in its very nature the suspension, not 

 only of one form, but of all forms of reflex selection between the 

 separated portions of the species. The importance of the cessa- 

 tion of natural selection in producing the different stages of the 

 degeneration of organs that are disappearing has been fully 

 discussed by Prof Romanes (see Nature, vol. xli. p. 437, and 

 previous communications there referred to), who points out that, as 

 the power of the special form of heredity by which any organ is 

 produced has been built up by the many generations of natural 

 selection that have acted in favour of the organ, so the gradual 

 weakening of that power follows the cessation of the natural 

 selection. Prof. Weismann seems to appeal to the same principle 

 when he attributes the disappearance of "rudimentary organs " 

 to the action of "panmixia," Now, in the cessation of reflex 

 selection which follows independent breeding, a similar principle 

 is introduced, and the inevitable result must be the weakening of 

 the power of heredity by which the portions of the species were 

 held in correspondence with each other before their separation, 

 I have elsewhere shown that separate breeding necessarily 

 disturbs unstable adjustments ; and we here see that the most 

 stable of the adjustments by which each part of a species is kept 

 in correspondence with every other part gradually becomes un- 

 stable under the continued influence of separation. Whenever 

 a species is divided into two portions that do not interbreed, the 

 four forms of reflex selection above described will cease to act 

 between the two portions, and they will continue in sexual, 

 social, physiological, and industrial harmony with each other, 

 only in so far as the force of the old heredity holds them to the 

 old standards. But the power of heredity in these respects will 

 in time fail, if reflex selection is entirely removed. If the 



NO. 1085, VOL. 42] 



