Nature of Heredity and Problem of Mechanism 323 



of a great intricacy and succession of transformations. From 

 this it follows that the "mechanism of heredity" would be the 

 materials and structures by which these transformations are 

 accomplished. To think of the hereditary substance, or the 

 "mechanism of heredity" as belonging to the germ-cells alone 

 is utterly unwarranted by the obvious facts, as everybody 

 must see who will reflect broadly and candidly on the sub- 

 ject. 



A truly objective study, consequently, of the mechanism 

 of heredity must be a study not only of the materials and 

 structures in the germ-cell stages, but in all stages what- 

 ever, by which the production of hereditary parts and attri- 

 butes is accomplished. 



In other words, a real study of biotic genesis, whether 

 the generating parts be hereditary or variational, that is, 

 like or unlike those of ancestors, must be first and essentially 

 by the methods of descriptive ontogenesis. The only direct 

 evidence we have or can have of the origin of a part or an 

 organ is the observed transformations by w^hich that part 

 or organ is produced from preceding parts, and the mate- 

 rials participating in the transformations are the hereditary 

 substances, if that term is to have any legitimate meaning. 



To illustrate: the lens of the vertebrate eye originates 

 from a patch of ectoderm exterior to the optic globe. The 

 optic globe itself arises by an outpocketing of the primitive 

 brain. Since both lens and globe resemble the corresponding 

 parts of the eye of ancestors near and remote, their develop- 

 ment comes under the principle of heredity ; and the ecto- 

 dermal patch giving rise to the lens, and the part of the 

 primitive brain giving rise to the optic globe are mechanisms 

 of heredity ; and the whole observable series of embryonic 

 parts which culminate in the completed eye are the only 

 direct evidence for the mechanism of heredity for the eye. 

 So is it with all biotic ontogenesis whatever. This brief 

 statement is in essence nothing more than the gist of the 



