EVOLUTIONARY MECHANISMS 3O3 



The evidence against the theory of the inheritance of acquired 

 characters is, in particular cases, very strong. Thus the docking of 

 sheeps' tails and circumcision of man have been carried on for hundreds 

 and perhaps thousands of years without producing any hereditable 

 effects. On the other hand, we can find plausible cases on the other 

 side. For example, the thickened skin on the sole of the human foot, 

 and the sternal and alar callosities of the ostrich^ seem to be directly 

 related to the pressure arising from the habitual positions of these 

 animals; the simplest hypothesis is that, originally, the thickenings 

 were reactions to pressure. But now the thickenings arise in the em- 

 bryo, before any pressure is exerted. They are therefore hereditarily 

 fixed, and it is argued that natural selection could not explain this 

 fixation since the callosities can be of no service to the embryos. The 

 conditions can, perhaps, be compared with the much wider class of 

 phenomena known as double assurance in embryonic development. 

 These are cases in which an organ is normally induced by an organizer 

 from competent tissue, but in which the organ can develop by self- 

 differentiation even if the organizer is removed. An example is the lens 

 in the frog, normally induced by the eye cup, but able to develop in its 

 absence. There is no obvious advantage in this, since it profits a frog 

 little if it retains its lens but loses its eye. But we know too little about 

 the genetic control of competence to assert that this exaggeration of 

 competence is useless. It may be that the evolution of a double assur- 

 ance mechanism is akin to the selection of genes with a margin of 

 safety adequate to deal with minor variations in developmental con- 

 ditions. 



Another phenomenon which is brought forward as evidence of 

 Lamarckism is the production by particular environments of forms 

 similar to varieties which are known to be hereditable. This is taken to 

 show that the inherited varieties originated as reactions to similar 

 conditions, perhaps very long continued. The data have recently been 

 summarized by Rensch^ from a faunistic point of view. There is how- 

 ever nothing particularly mysterious in the fact that hereditable varie- 

 ties can be imitated by the action of environmental agents during 

 development. The phenomenon has been discussed (p. 191) and we 

 saw that the environmental action can be explained in terms of the 

 genotypic constitution of the organism, while there are no grounds for 

 suggesting a relation in the reverse direction and trying to explain the 

 genetic control as a result of the environmental effect. 



^ Duerden 1920. ^ Rensch 1929, 1936. 



