Nature of Heredity and Problem of Mechanism 311 



theories, is certainly non-existent so far as germ-cells are 

 concerned in many of these species. 



The aim so far has been merely to fix attention on the 

 tendency of present day theorizing on heredity to restrict 

 the conception to phenomena presented in reproduction 

 through the instrumentality of germ-cells of two sorts, male 

 and female. It is hoped that this brief statement of the 

 tendency has revealed its unwarrantableness to the uncom- 

 mitted reader. But since our general enterprise requires us 

 to perceive its fallaciousness we shall have to examine it in 

 considerable detail. 



Unwarrantable Tendency to Restrict Heredity to Adult 



Characters 



Another serious shortcoming in the way problems of 

 heredity are treated must be noticed. Reference is made 

 to the practice particularly^ by recent geneticists of treating 

 heredity as though it primarily concerns only the attributes 

 of adult organisms. To be sure both popular and scientific 

 observation depart to some extent from this rule. Notice 

 is often taken of the resemblance to their parents of young, 

 even newly born humans and lower animals, but even here 

 the basis of comparison is the adult, the parent. Rarely are 

 resemblances of the embryo in its many stages of develop- 

 ment to the embryos of its parents at the corresponding 

 stages of their development thought of as phenomena of 

 heredity. Raymond Pearl, in his Modes of Research in Gen- 

 etics^ has pointed out this defect more comprehensively than 

 any other writer so far as I know. Under the topic, "The 

 Embryological ^Methods of Research in Genetics," after 

 remarking that embryology has been cultivated mainly for 

 its own ends, he writes, "Only in a relatively small portion 

 of instances, has it been directly and purposefully used as 

 a mode of research in genetics. Yet embryology is the scir 



