RELATIVE GROWTH AND RECAPITULATION 239 



we may suggest, rather to the fact that a large adult chela 

 must begin its existence of small relative size, and that the 

 form of the small chela has been moulded by natural selection 

 into the female-type or feeding chela. A similar reasoning 

 applies to the male type of juvenile female abdomen in all 

 Brachyura : this affords no presumption whatever that 

 ancestral female crabs when adult ever possessed an abdomen 

 as narrow as that of the existing male. The presumption is 

 quite different — that during Brachyuran evolution the male 

 abdomen has been narrowed, the female broadened, and that 

 it has proved an ontogenetic convenience to produce the 

 abdomen at its first appearance, when it is relatively small, 

 in a form similar to that of the male, from which the female 

 type is derived by differential heterogony. 



Numerous other examples could be given of this distinction 

 between von Baer's and Haeckel's laws ; but enough has been 

 said to show the importance of the distinction, and the reason 

 for the greater universality of von Baer's generalization. 



Further, it is in general clear that rate-genes may mutate 

 in either a plus or a minus direction, either accelerating or 

 retarding the rates of the processes they affect. In the former 

 case the effect will be in certain respects at least recapitulatory, 

 since a condition which used to occur in the adult is now run 

 through at an earlier stage. In the latter case, the effect 

 will be anti-recapitulatory, since a condition which once 

 characterized an earlier phase of development is now shifted 

 to the adult phase. Neoteny is the most striking example 

 of this effect ; but we may also get single characters behaving 

 in this way. When this occurs, previous adult characters, 

 though still in a real sense potentially present, never appear 

 because their formation is too long delayed : they are lost 

 to the species by being driven off the time-scale of its develop- 

 ment. For examples of this, notably in Helix, see the discus- 

 sion in Ford and Huxley, 1927. (Fig. 104.) 



De Beer (1930), in his resume of this and kindred subjects, 

 has given the useful term paedomorphosis to this type of effect. 

 Bolk (1926) has drawn attention to the importance of such 

 effects in the evolution of man, summing up the result as a 

 process of ' fetalization ', since in many respects post-natal 

 or adult man resembles the fetal stages of apes. See also 

 Kieslinger (1924). 



Such phenomena are unintelligible on the Haeckelian 

 doctrine. But they immediately fall into place when it is 



