No. 107 1, Vol. 42] 



NATURE 



29 



fertility depended on such delicate adjustment of the male and 

 female elements to each other that, unless constantly kept up by 

 the preservation of the most fertile individuals, sterility is always 

 ready to arise. ... So long as a species remains undivided, 

 and in occupation of a continuous area, its fertility is kept up by 

 natural selection ; but the moment it becomes separaled, either 

 by geographical or selective isolation orby diversity of station or 

 of habits, then, while each portion must be kept fertile inter se, 

 there is nothing to prevent infertility arising between the two 

 separated portions " (p. 184). Here is an application of the 

 principle of segregation, or of like to like in groups that do not 

 <;ross, in which indiscriminate .reparation is followed by increas- 

 ing divergence in the different portions, not because they are ex- 

 posed to different environments, not because there is any 

 advantage in such divergence, not because there is any need that 

 the function should be performed more perfectly in one portion 

 than in the other, but because intergeneration, which is the 

 principle by which correspondence of function is secured, has 

 been suspended for some generations ; and, in the absence of 

 intergeneration, neither natural selection nor any other principle 

 is capable of preserving complete correspondence. In organisms 

 that reproduce sexually, the causes of divergence are many, 

 though they may all be classed as causes of segregation, while 

 the causes of correspondence with variation, whether of functions 

 or of structures, are causes of intergeneration between partial 

 segregations. If the environments surrounding the isolated 

 portions are the same, the use of the environment, and therefore 

 the forms of natural selection, may become divergent ; if the 

 use continues unchanged, some useless divergence in the method 

 of securing the use may appear. Or, if all the relations to the 

 environment, whether useful or useless, remain unchanged, " the 

 adjustment of the male and female elements to each other" are 

 liable to become slightly divergent, producing n.utual infertility. 

 Or the preference of the sexes for certain shades or arrangements 

 of colour in their mates may become slightly different. Through 

 some slight difference in the hereditary elements, distributed in 

 each separated portion at the first, one or all of these causes of 

 accumulated divergence may be introduced. I think it is evident 

 that we have here a general principle, which is as applicable to a 

 wide range of divergences as it is to the divergence that produces 

 mutual infertility and sterility. 



The context shows that the prominent idea in Mr. Wallace's 

 mind was divergence in the adjustment of the male and female 

 elements through correlation with " some diversity of form or 

 colour," resulting from divergent forms of natural selection, 

 that had been induced by exposure to " somewhat different con- 

 ditions of life." But if the reasoning is correct in the sentences 

 I have quoted, it gives an explanation of similar divergences 

 when the separated portions are exposed to the same environ- 

 ment, and whirc there is no possible advantage to be gained 

 by divergence. This is one of the principles I have used 

 in the explanation of the divergences of Sandwich Island 

 land mollusks ; and I think that in the earlier stages of the 

 development of infertility between allied forms it is often the 

 only explanation that is applicable. It should, however, be 

 remembered that, for divergence of this kind, it is not always 

 necessary that the isolation should be either complete or very 

 long continued, and that, when the forms that are not fully 

 fertile with each other meet and more or less commingle, there 

 is, through the very laws of propagation, without any aid from 

 natural selection, a constant increase in the ratio of the pure 

 breeds to the mongrels, and an accumulating intensity in the 

 segregative instincts and the physiological incompatibilities. 

 As this point has been fully discussed in my paper on " Diver- 

 gent Evolution," I do not need to enlarge on it here (see 

 Linn. Soc. Journ., Zoology, vol. xx. pp. 246-72). 



There is, however, another phase of the subject which is in- 

 dicated by Mr. Wallace's suggestion that infertility depends on 

 "such a delicate adjustment " that it is more easily affected by 

 isolation than some other adjustments. This is, I think, a very 

 interesting point, as it suggests how it is that, in some cases at 

 least, phy.-iological divergence of this kind is one of the first 

 forms of divergence that arises. But in some species other ad- 

 justments seem to be more delicate than this, and therefore more 

 easily disturbed ; while in others, several sets of adjustments, as 

 colours and other recognition marks, with the preferences that 

 correspond, and the habits of feeding and defence are in a state 

 of equilibrium, the stability or instability of which is about the 

 same as of that which determines the relations of the male and 

 female elements. In this last class of cases, several forms of 



divergence may arise during the same stage of development, and 

 that too when the isolated portions are exposed to the same 

 environment. In some species a large number of characters 

 are in a state of unstable adjustment. As Prof. Lankester has 

 suggested near the close of his review of Mr. Wallace's book, this 

 cause of divergence seems to be specially operative in the case 

 of human faculties. But variability with plasticity of type is 

 not the only condition that affects the stability of segregated 

 portions of a species. Other things being equal, a single pair of 

 any species is much less likely to represent the average of all 

 the characters of the species than a million pairs. This con- 

 sideration throws light on the comparative lack of divergence 

 between the land animals of England and those of Ireland, 

 which lack has been referred to by Mr. Wallace as an objecti )n 

 to my theory. In this case, many millions of some of the species 

 were probably existing in each district at the time of the 

 separation. As Prof Lankester has pointed out, the representa- 

 tives of the human species in the two districts have somewhat 

 diverged ; and the probability is, that if we were equally ac- 

 quainted with the other species, we should find other examples 

 of divergence in minor points. If the isolation is made more 

 complete, and is longer continued, I believe the divergence will 

 gradually become more apparent. 



Mr. Wallace has mentioned another class of divergences 

 which are best explained by the principle we are now considering 

 — as he seems to have apprehended, th )Ugh the process is not 

 stated here as clearly as when discussing the divergences that 

 produce infertility. The passage is as follows : — " The enor- 

 mously lengthened plumes of the bird of paradise and of the 

 peacock must be rather injurious than beneficial in the birds' 

 ordinary life. The fact that they have been developed to so 

 great an extent in a few species is an indication of such perfect 

 adaptation to the conditions of existence, such complete success 

 in the battle of life, that there is, in the adult male at all events, 

 a surplus of strength, vitality, and growth-power which is able to 

 expend itself in this way without injury. That such is the case 

 is shown by the great abundance of most of the s-pecies which 

 possess these wonderfil superfluities of plumage. . . . Why, in 

 allied species, the development of accessory plumes has taken 

 different forms, we are unable to say, except that it may be due 

 to that individual variability, which has served as the starting- 

 point for so much that seems to us strange in form, or fantastic 

 in colour, both in the animal and vegetable world" ("Dar- 

 winism," p. 293. The italics are mine). 



It is no small gratification to me that Mr. Wallace has found 

 this principle of unstable adjustment worthy of application to 

 two important classes of divergences ; and that, in the case of 

 one of these classes, he has recognized that correspondence in 

 such adjustments cannot be continuously maintained between 

 the isolated portions of a species. I, moreover, have some hope 

 that, when he understands the relation in which instability and 

 isolation stand to each other in my theory, he will admit that 

 it throws some light on the remarkable divergences of Sandwich 

 Island land mollusks. The subject was cnly incidentally 

 touched upon in my paper on " Divergent Evolution through 

 Cumulative Segregation," but will be more fully discussed in a 

 supplemental paper on " Intensive Segregation." 



26 Concession, 0.saka, Japan. John T. Gulick. 



Coral Reefs, Fossil and Recent. 



Many Alpine geologists believe the hmestone and dolomite 

 mountains which form so peculiarly beautiful and interesting a 

 part of our Eastern Alps to be in great part composed of Triassic 

 coral reefs. If this be so, their geological structure must neces- 

 sarily contribute much towards elucidating the discussion concern- 

 ing the origin of atolls and other forms of recent coral reefs. In 

 this discussion, which has chiefly been carried on in England, 

 the structure of our Triassic limestone mountains has been left 

 out of account in a manner very surprising to me, considering 

 that authorities like Richthofen and Mojsisovics have declared 

 them to be remnants of coral reefs. 



I have made a number of Alpine ascents in the dolomites of 

 South Tyrol, chiefly in the districts of the Hohlensteinthal, 

 Primiero, and the Langkofel, and have satisfied myself that the 

 theory of the coralligenous origin of great part of these moun- 

 tains is the only one which will explain the position and nature 

 of the rocks composing them. 



Not only do we obseive in many places the massy dolomite 

 alternating at its margin with sedimentary deep-sea deposit of 



