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SCIENCE-GOSSIP. 



it was one of Darwin's conclusions from his 

 experiments on cross-fertilization, that crosses 

 between varieties are more vigorous than others. 

 Thus, with regard to plants, he remarks : "I 

 suspect that it must arise from the pollen of a 

 distinct variety having a prepotent effect over a 

 flower's own pollen." 



This, however, applies to domesticated species, 

 and Dr. Romanes believed that there is a difference 

 between these and natural species in this respect. 

 It would be strange if variation among domestic 

 species were normally accompanied by increased 

 fertility, while in nature it went hand in hand with 

 infertility. 



The great difficulty in physiological selection 

 seems to be a variant of the problem, how the 

 horse is to feed while the grass grows ; or, how a 

 variation can be preserved and fostered by natural 

 selection while too insignificant to be useful. How, 

 that is to say, is the new variety to be kept 

 separate while the bar of sterility is being forged 

 from some slightly diminished cross fertility ? 



If a number of individuals in a species could be 

 assumed to acquire at once an absolute sterility 

 with the rest, and retain perfect fertility, inter se, 

 then we might accept physiological selection as a 

 vera causa in the evolution of species : a small 

 degree of cross-infertility is useless. 



Another point in which Dr. Romanes seems to 

 differ from other Darwinians, is in believing that 

 isolation alone can produce divergence when a 

 small part of a species is separated from the rest. 

 The mean or average character of the separated 

 portion, differing slightly from the remainder, will, 

 when cross-breeding has reduced it to uniformity, 

 have diverged a little from that of the parent stock. 

 Such being the case, we should expect to find 

 small isolated colonies of a race varying from the 

 main body. There may be examples to support 

 this view, but there are certainly an overwhelming 

 number against it. For example, a number of 

 colonies of Arctic plants are found on the higher 

 mountains of Wales and Scotland, where they have 

 been separated from the main bodies of their 

 species since the close of the Glacial period 

 Similarly isolated colonies of butterflies are also 

 known. Most interesting is the case of a beetle, 

 Chrysomela cerealis, found on Snowdon, and no- 

 where nearer than the cornfields of France. 

 Again, we have the case of the rabbits and sparrows 

 imported to Australia and New Zealand, and 

 English weeds introduced into America, all in small 

 numbers, and all without any indications of the 

 formation of new species. The colony of South 

 European plants in the south-west of Ireland, and 

 of many aliens and colonists in England, are 

 further examples. Indeed, as we reflect, the 

 adverse cases come crowding in upon us — they are 

 without number. Dr. Romanes himself, with 



Darwin-like candour, brings forward several 

 interesting examples which he admits are against 

 his view. 



Wallace, in criticising Romanes' theory, quotes 

 the case of Ireland as showing that isolation has 

 not produced divergence of species as it ought, if 

 it were a vera causa. Ireland, he says, furnishes 

 " an excellent test case, for we know that it has 

 been separated from Britain since the end of the 

 Glacial epoch .... yet hardly one of its 

 mammals, reptiles or land molluscs has under- 

 gone the slightest change." To this Dr. Romanes 

 replies that Ireland is not an excellent test case for 

 the efficiency of indiscriminate isolation as a factor 

 in evolving species, since the potency of isolation 

 varies inversely as the number of individuals 

 separated. When Ireland was severed from 

 Britain there were too large numbers of the 

 various species in it to allow of divergence from 

 the parent type. Such is the answer. If we want 

 a good test case we must choose one where only a 

 small number of the species have been separated. 

 Yet if we were to refer to the various small 

 islands off the coast of Britain where species 

 have long been isolated in small numbers, we 

 should, presumably, be told that they were too 

 near the shore. We finally gain the impression 

 that the places where species can be manufactured 

 are few and far between, if not actually in nubibus. 



Whether Ireland be a good test case or not, 

 we must be permitted to say it comes a little 

 strangely from Dr. Wallace. For surely on his 

 own views, equally with those he is criticising, 

 there ought to be divergence of species in Ireland. 

 There is difference in environment. For the 

 climate is not the same ; there is a difference in the 

 assemblage of animals (absence of reptiles, etc.) 

 and of plants (presence of foreign species, and 

 absence of others occurring in Britain), while the 

 soil on the whole is also different. The reason for 

 the lack of change is given by Wallace as follows : 

 " That changes have not occurred through natural 

 selection is, perhaps, due to the less severe struggle 

 for existence, owing to the smaller number of 

 competing species." 



If there had been changes, would not Wallace 

 have claimed them for natural selection ? Ireland 

 seems, indeed, an excellent test case for the theory 

 of natural selection in general. For if in the period 

 of So.ooo years which some tell us have elapsed 

 since the close of the Glacial period, it has done 

 nothing, how could even the millions demanded by 

 geologists and biologists be held sufficient to evolve 

 the organic world of to-day ? 



There are, says Dr. Romanes, two great diffi- 

 culties for the theory of natural selection. First, 

 natural selection alone can only lead to monotypic 

 evolution, that is, it can only gradually modify one 

 species into another : it cannot split up a species 



