CHAPTER VII 



OTHER FACTORS OF EVOLUTION 



IT is important to bear in mind what has hitherto been said 

 as to the comparatively trivial character of the differences that 

 mostly go to separate one species from another, and the common 

 absence of any evidence of ' usefulness ' in such characters. But 

 if we regard ' organic polarity ' and changes in molecular consti- 

 tution as lying at the root of the origin of ' sports ' and as the 

 cause of ' mutation ' generally, there would no longer be any 

 discrepancy in many cases, since we are expressly told by de Vries 

 that the changes due to mutation " occur at random — that is, in 

 the most diverse directions." 



In illustration of the comparative ease with which changes are 

 brought about in the specific characters of some organisms, refer- 

 ence may be made to the results of experiments with the larvee of 

 certain butterflies. It is now well established that the colours and 

 markings of many of these latter, together with the size, and to a 

 less extent, the shape of their wings, are largely dependent upon 

 the conditions as regards temperature and light under which the 

 caterpillars and pupae have been reared. Hence the phenomena 

 of ' Seasonal Dimorphism,' in which one form of butterfly is bred 

 in the Spring, and a different one later on in the Summer. But 

 the experiments of Merrifield ' and of Weismann have shown 

 that one of the two seasonal forms may be bred from the larvae 

 of the other form by simply altering the temperature under which 

 the larvae are reared — the changes, in fact, though slighter in 

 amount, are produced just in the same way that changes in the 

 colour and forms of crystals are brought about (pp. 45, 63). 



There are also other sets of changes of like kind, lying outside 

 the pale of utility, and yet apparently connected with one another 

 in some mysterious manner : I allude to those which were grouped 



• " Nature," Dec. 23, 1897, p. 184. 



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