IV NUTRITION AND PIIYLETIC GROWTH 



109 



terrestrial animals, have been compelled to retain their ^A\\% 

 and remain aquatic by being prevented from leaving the water, 

 or at least from breathing much air directly, as for instance 

 when the vessel in which they were contained was covered 

 by a cloth. The success of such experiments shows Ijy 

 itself the important part which the direct influence of external 

 conditions plays in the modification of organisms. 



The fact, if proved, that insufficient nourishment can cause 

 an organism to remain at a low stage of its normal develop- 

 ment, i.e. can retard its phyletic growth, just as bad nourisli- 

 rnent hinders the individual growth, would go very far to 

 justify the explanation of the evolution of forms as growth. 



Indeed, nature affords instances which make it a priori 

 probable that inadequate nutrition may have this effect. 

 It is well known that the cockchafer takes three years to 

 develop in the south of Germany, four years in the 

 north. In South Germany every third year is a cockchafer 

 year, in the north every fourth. But it happens in South 

 Germany, after a long inclement winter, that in one region or 

 another even there the development is retarded by a year ; 

 and accordingly in localities which are adjacent but sheltered 

 in different degrees the cockchafer years are not tlie same. 

 In exceptionally warm summers even in the north the 

 cockchafers appear a year earlier ; these are the beetles which 

 fly in August and September. 



The last period of its life beneath the earth the larva passes 

 in the chrysalis state. The change into the chrysalis takes 

 place usually in the early summer of the year which precedes 

 the appearance of the beetle. The variations of temperature 

 accelerate or retard the chrysalis stage. As the variation of 

 temperature, however, shortens or prolongs the feeding-time of 

 the larva, it may be assumed that the change of tlie voracious 

 larva into the chrysalis stage is conditioned partly by 

 nutrition. 



