LAW OP DEVELOPMENT KNOWN AS VON BAEE's LAW. 85 



ment seems to be the rule, the former the exception. How 

 are we to account for the exception ? The key to the position 

 is, in my opinion, to be found by comparing the conditions of 

 larval and embryonic development. In larvae the organs are 

 functional and the animal is getting its own living during the 

 development, whereas in embryos the development takes place 

 under the protection of egg membranes, the pupal case, or the 

 uterine wall, and the organs are for the most part functionless, 

 special arrangements being made for the supply of nutriment. 

 These two developments have generally not been properly dis- 

 tinguished by naturalists writing on this subject. 



In embryos the organs are for the most part functionless 

 and without relation to the maintenance of life ; consequently 

 there is nothing to counteract the tendency to the appearance 

 of a variation at all stages in the life of an organ. In larvae, 

 on the other hand, the organs are functional and the con- 

 ditions of life may be different from those of the adult. They 

 have to maintain themselves during the various phases of their 

 development ; consequently if a variation of an organ at one 

 stage is injurious to the same organ at a previous or subsequent 

 stage, it will be eliminated at the stages at which it is injurious. 

 In this way, as will be readily seen, natural selection will com- 

 pel the limitation of variations in an organ to particular stages 

 in the development of that organ ; the power of natural selec- 

 tion will inevitably prevent a variation useful at one stage 

 from affecting another stage of the same organ in which its 

 presence would be injurious to the larva. Thus there must be 

 in larvae a tendency to the inheritance of variations at corre- 

 sponding periods, and to the elimination of them at other 

 periods when they would be harmful to the organism. 

 Thus it must happen that if variations occur which enable the 

 adult to change its condition of life, and if at the same time 

 the old habits of life are retained by the last larval stage, then 

 the old arrangement of organs will be retained by the larva. 

 In this way, as the adult form gradually progresses in evolution, 

 not only one but a whole series of larval stages might become 

 established, each one being based upon some ancestral stage of 



