LAW OF DEVELOPMENT KNOWN AS VON BAER’S LAW. 45 
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 larve 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 larvee 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 
