92 SECTIONAL ADDRESSES 



ceased to have litters and began to bring forth a single young at a birth 

 that the further evolution of man became possible. 



The slowing of human development further had numerous corollaries. 

 The typical adult human condition of hair on the head but almost 

 complete absence of hair on the body, the hymen of the human female, 

 and the smooth orthognathous form of the human face and skull appear 

 to be based upon characters automatically transferred from earlier to 

 later stages of the life-cycle. 



This general slowing down of man's post-natal development is doubtless 

 due in part to its possessing selective advantage. But, as Haldane points 

 out, it may also be in part the indirect carry-over from a slowing of pre- 

 natal development. In the circumstances of primitive sub-man a foetus 

 is on the whole better nourished and less exposed to danger than a new- 

 born infant, so that pre-natal slowing is here as advantageous as pre-natal 

 acceleration in a polytocous mammal. 



This prolongation of a more protected early phase may also apply to 

 the larval period, for instance in insects with their ccenogenetic larvae, 

 which are often highly adapted to their secondary mode of life. One 

 need only think of the mayfly with its imaginal phase reduced both in 

 structure and in duration. 



Sometimes this reduction is carried to its logical extreme and the adult 

 phase is wiped out of the life-history by neoteny. This has demon- 

 strably occurred in various beetles, and in the axolotl. It has probably 

 taken place in ourselves as well, with the heavy brow-ridges and pro- 

 truding jaws of our ancestors. 



Haldane in an interesting paper discusses these and similar phenomena 

 from the standpoint of the time of action of the genes controlling them. 

 A more comprehensive view, however, would include as still more im- 

 portant the genes' rate of action. 



A large number (possibly the majority) of genes exert their effects 

 through the intermediation of a process operating at a definite rate. 

 The speeds of processes which such rate-factors control are not abso- 

 lute, but relative — relative to the speeds of other processes of develop- 

 ment and of development in general. It is also found that a decrease 

 in rate of process is in general accompanied by a delay in the time of its 

 initial onset, and vice versa. Furthermore, such processes do not neces- 

 sarily continue indefinitely. Often they reach an equilibrium ; when this 

 is so, the level of the equilibrium is correlated with the rate of the process. 

 This is so, for instance, with eye-colour in Gammarus, and probably in 

 man. In addition to such rate-factors, others are known which appear 

 only to affect the time of onset of a process and not its rate. 



Attempts have been made by representatives of the Morgan school 

 to minimise the importance of these discoveries, by asserting that they 

 constitute only a redescription of old phenomena and add nothing truly 

 new. On the contrary, I would maintain that they are of first-rate 

 importance. I need not go into their bearings upon physiological genetics. 

 Here we are concerned with their evolutionary implications. 



In the first place, since rate-genes are common, it is a legitimate pro- 

 visional assumption that the rates of developmental processes in general 

 are gene-controlled. Further, the simplification introduced into an 



