SCIENCE-GOSSIP. 



3ii 



seems to be absolutely necessary for the theory of 

 physiological selection that the cross - infertile 

 individuals should breed together and not with 

 the parent stock, it seems to be no part of the 

 theory that they necessarily do so, nor is there any 

 evidence forthcoming from the facts of nature that 

 such is the case. No one, we believe, has con- 

 tended that the fact of potentially lessened fertility 

 between two individuals would prevent them 

 breeding together. 



Again, we must ask, is cross-infertility of the 

 requisite kind at all common ? Different degrees 

 of fertility are of course frequent ; but does it often 

 happen that an individual A is perfectly fertile with 

 another B, and comparatively sterile with C, while 

 C again is perfectly fertile with D ? This is what 

 would be required, but evidence is not forthcoming. 

 Infertility between species and between varieties 

 of the same species is brought forward, and this, 

 in the eyes of some, is enough to prove the existence 

 of the other. Again. Dr. Romanes tells us that : 

 " The importance of physiological isolation, when 

 once fully developed, cannot be denied, for it is 

 evident that if such isolation could be suddenly 

 destroyed between two allied species occupying a 

 common area, they would sooner or later become 

 fused into a common type." 



Here Dr. Romanes does not seem to me to 

 distinguish clearly between infertility and disinclina- 

 tion to breed together. Mutual fertility would not 

 in itself induce two species to breed together unless 

 the bar which at present prevents them even trying 

 to come together were removed likewise. The 

 difficulty in breeding mules is not sterility, but in 

 overcoming the natural disinclination of the two 

 species to come together. Infertility is not the 

 reason why species do not attempt to breed to- 

 gether. There is disinclination, or some physical 

 bar. Again, before any section of a species rendered 

 cross-infertile can develop into a new variety there 

 must be also change of form. Now when we look 

 to our domestic breeds we find that great change 

 of form has not been accompanied by infertility. 

 Hence we must suppose it a mere coincidence if 

 cross - infertility in nature is accompanied by 

 departure from the normal type. This makes 

 difficulties for physiological selection. In tin: fir si 

 place, the cross-infertility and mutual fertility has 

 to appear simultaneously in a number of Indi- 

 viduals Before a new species can an e there 

 must be outward variations among the e, 1101 



.iting in the 

 n '.( the character! of the new species. 



I urlher. if 111 the dot 



not necessarily accompanied by In 

 fertility, neither 1 that infertility is 



likely t'i l/e a/' ompanied l.y divi ■ ' e 

 Again, st- • entually arises in cast 



. yet was not the cause -.1 tip- 



isolation or differentiation. Dr. Romanes, indeed, 

 hints that species separated geographically will be 

 found to be more frequently cross-fertile than those 

 living together. No proof of this, however, is 

 given. Darwin's opinion on this point was given 

 thus : " Species which have never co-existed in the 

 same country, and which therefore could not have 

 received any advantage from having been rendered 

 mutually infertile, are generally sterile when 

 crossed." 



What positive evidence, let us now ask, does the 

 author bring forward in support of his view ? It 

 must be confessed that what is offered is neither 

 very abundant, nor very conclusive. It is, more- 

 over, of an indirect kind, chiefly consisting in this, 

 that certain facts are more easily explained by it 

 than on the assumption that natural selection has 

 been the sole factor in species making. Thus, the 

 fact that large areas are better manufacturers of 

 species than small ones is a difficulty without 

 physiological selection, on account of the swamping 

 effects of intercrossing on such larger areas. 

 Darwin, by the way, explained this by the fiercer 

 competition. Again, the fact of closely allied 

 species living together is claimed to be more 

 in accordance with physiological than natural 

 selection. 



Touching Nageli's examples of closely-related 

 plants growing in close proximity, Romanes 

 writes: " But now, if this bar is thus necessary 

 for preserving the specific distinctions when they 

 have been fully developed, much more must it 

 have been so to admit of their development." Yet 

 when the bar was most necessary it was feeblest ! 

 The bar is necessary to separate them, and yet 

 it can scarcely be said to exist until they are 

 separated. 



Dr. Romanes points out that on his theory 

 varieties ought to show cross-infertility, whereas 

 the theory of natural selection has no reason 

 to anticipate such a state of things. As to fact, 

 many varieties of plants are found growing close 

 together, and yet retaining their independence. For 

 evidence of the actual existence of such infertility 

 1 )r. Romanes quotes only the experiments of 

 M. A. Jordan. This French observer found that 

 in many hundreds of cases varieties came true to 

 •''il, and were cross-sterile inter sc. Even with all 

 this evidence, it remains an assumption that the 

 Sterility was the cause of the variations. Can we, 

 moreover, accept the evidence given above as proof 

 of general sterility between varieties? It is from 

 a single set of experiments mode some twenty 

 three years ago, and there musl surely I"- much 

 evidence available (or or against Ihe view. 

 ■ I, as it Romanes tells us, tin: experi- 

 ments were made with a special object in view, 

 viz,, i" pro vi- that all such varieties were 

 separately created species On the othei band, 



