106 TEST CASES [ch. xi 



skin, as apparently have other northern tribes that migrated into 

 India. The principal change that he would undergo would be a 

 gradual physiological adaptation to warmer climates. 



Many, if not most or even all, of the characters of distinction 

 that mark families, sub-families, and even smaller groups, are 

 such that they can have no serious value upon the physiological 

 side, which is the only one that matters from the point of view of 

 natural selection or gradual adaptation. Only upon things with 

 functional value or disadvantage can natural selection operate, 

 and, as has frequently been pointed out by the writer and others, 

 its important work seems to be the killing out, probably rapidly, 

 of any variation definitely disadvantageous, though even here, as 

 the struggle for life is mainly among seedlings, disadvantageous 

 characters that only appear late in life may quite well survive. 

 There is no doubt that natural selection would encourage the 

 success of a new and improved form that had just arisen, but 

 there is no evidence that it can continue to call up small variations 

 or mutations always in the right direction, or that it can pass the 

 rough and ready line of distinction that exists between species, 

 that of mutual sterility, unless some mutation should happen 

 that will do so. But work of this kind will not ensure progress 

 such as seems to be the mark of evolution in general. Suppose a 

 whole family to possess a septicidal capsule, or diplostemonous 

 stamens. There is no evidence to show that there is any physio- 

 logical value attaching to this possession, which in any case only 

 appears in later life. One cannot imagine natural selection killing 

 out a member of the family that had adopted (or was varying — 

 if it could so vary — in the direction to adopt) a loculicidal capsule, 

 or obdiplostemonous stamens, or was even going so far as a septi- 

 fragal capsule. The family constancy of the capsule or the stamens 

 must be due to inheritance from a common ancestor. But how, 

 under selection, did the ancestor of one family obtain one kind of 

 capsule or stamens, of another family another? With the recent 

 revival of natural selection, there has been a recrudescence of the 

 idea that characters that are of no physiological value tend to be 

 very variable, but if so, why are family characters less variable 

 than generic and specific, though they are admittedly of less 

 physiological value? 



Plants, animals, and man alike tend to produce so many off- 

 spring that, in a short time, but for various unfavourable condi- 

 tions, there would not be room for them upon the surface of the 

 earth. The illustration taken from the rapid multiplication of the 



