D.— ZOOLOGY 93 



analysis of development by the concept of relative rates of processes 

 makes it desirable to try this key first of all when attacking any problem 

 involving development. 



It then provides a great simplification of the facts of recapitulation 

 and anti-recapitulation. Whenever the rate of a process is correlated 

 with time of onset and final equilibrium-level, a mutation causing an 

 increase in rate will produce recapitulatory phenomena. It will drive 

 the visible onset of the process further back in ontogeny, will add a new 

 ' hypermorphic ' character at the end of the process, and will cause all 

 the steps of the original process to be recapitulated, but in an abbreviated 

 form, during the course of the new process. This will account, for in- 

 stance, for many of the recapitulatory phenomena seen in the suture lines 

 of ammonites. 



Conversely, a mutation causing a decrease in rate will have anti- 

 recapitulatory effects. It will prolong the previous phase longer in ontogeny, 

 it will not only slow the process down but stop it at a lower level of 

 completion, and it will remove certain previous adult characters and push 

 them off the life-history. Many of the phenomena of so-called ' racial 

 senescence ' in ammonites, including the gradual uncoiling of the shell, 

 may be due to phenomena of this type. 



As de Beer has pointed out, when ccenogenetic changes occur in the 

 embryo or larva, the adult remaining unchanged, neither palasontology 

 nor comparative anatomy would register any phylogenetic advance. But 

 if now neoteny or fcetalisation occurs, the old adult characters may be 

 swept off the map and be replaced by characters of a quite novel type. 

 This process he calls clandestine evolution. Garstang has suggested 

 that it has operated on a large scale in the ancestry of vertebrates and of 

 the gastropods. 



A clear-cut small-scale example comes from the snail Cepea. Its 

 non-banded varieties are produced not because their genes cause the 

 total absence of pigment, but because they slow down pigment-formation 

 and delay its visible onset relatively to general growth, to such an extent 

 that growth is completed before any pigment can be formed. 



This is a comparatively unimportant effect ; but when major pro- 

 cesses are affected such as metamorphosis, sexual maturity, or general 

 rate of growth or development, the results may be far-reaching. Paedo- 

 genesis is caused by relative acceleration of the processes leading to 

 sexual maturity. Neoteny in the axolotl and presumably in insects is 

 due to the slowing down of the processes leading to metamorphosis. 

 The condition seen in man should not strictly be called neoteny, but rather 

 fcetalisation, or perhaps juvenilisation : this would seem to be produced 

 by a general slowing of developmental rate, relative both to time and to 

 sexual maturity. 



The existence of rate-factors has an important bearing upon the problem 

 presented by apparently useless characters. For alterations in the rate 

 of a process will often automatically produce a number of secondary 

 and apparently irrelevant effects. Numerous examples of such ' corre- 

 lated characters,' as Darwin called them, are now known. 



I will take a simple example from Gammarus. Here, the depth of eye- 

 colour depends upon the rate of deposition of melanin. But it depends 



