EDITOR'S INTRODUCTION xiii 



different, the animal has no fixed form. This is frequent in 

 Crustacea and Insects, and may even occur in mammals, as 

 Hinton has shown for voles. This obviously demands a 

 revision of certain taxonomic ideas on the value of precise 

 measurements of proportion. 



The fact, first emphasised by Goldschmidt, that Mendelian 

 factors frequently act by altering the rates at which develop- 

 mental processes occur and the times at which they begin and 

 end, rather than effecting qualitative changes ab initio, also 

 bears on the problem. The eye of Gammarus is first 

 scarlet, then darkens (at different rates according to the 

 genes present) to or towards black. This is no proof that 

 the ancestral eye was red, but depends on the physiology of 

 melanin-deposition. Bolk, as a result of morphological 

 study, has shown how frequently characters of early stages 

 become prolonged into later life in the course of evolution ; 

 his analysis enlarges the old concept of neoteny, and shows 

 how much more common it is than usually supposed. This, 

 however, is what the developmental physiologist would 

 expect. If the time of appearance and relative importance 

 of an organ depends upon the rate of some process, we are 

 just as likely to have that rate altered in one direction as in 

 the other, and therefore just as likely to have an embryonic 

 character spread on to later stages as an adult character 

 pushed back into earlier stages. The latter is recapitulation, 

 the former the reverse ; and both depend, not upon some 

 mysterious evolutionary urge, but upon simple develop- 

 mental laws. 



Other cases of recapitulation also become more intelligible 

 in terms of other aspects of Entwicklungsmechanik. Why, 

 for instance, are notochord, gill-slits, and arterial arches of 

 amniote vertebrates recapitulated, while their limbs never 

 recapitulate fins, and their gill-slits never recapitulate gills ? 

 The answer seems simple. The recapitulated ancestral organs 

 are necessities, as formative stimuli, for the production of 

 adult structures ; the non-recapitulated ones are not. 



" Racial senescence," so-called, is another morpholo- 

 gical-evolutionary concept which looks very different when 



