132 Heredity and Environment 



It must be said however that there are biologists who still re- 

 fuse to believe that heredity is associated with any particular cell 

 substance, while many others who would grant this are not yet 

 ready to admit that there are particular units or genes which are 

 concerned in the production of particular characters. However 

 anyone who will examine at first hand the evidences in favor of 

 this cannot fail to be impressed with its importance, and no one 

 has proposed any other hypothesis that is at all satisfactory. But 

 whether we assume the existence of these units or not we know 

 that the germ cells are exceedingly complex, that they contain 

 many visible units such as chromosomes, chromomeres, plasto- 

 somes and microsomes, and that with every great improvement 

 in the microscope and in microscopical technique other structures 

 are made visible which were invisible before, and whether the par- 

 ticular hypothetical units just named are invisible or not seems to 

 be a matter of no great importance, seeing that, so far as the 

 analysis of the microscope is able to go, there are in all proto- 

 plasm differentiated units which are combined into a system; in 

 short, there is organization. 



6. Heredity and Development. The germ cells are individual 

 organisms and after the fertilization of the egg the new individual 

 thus formed remains distinct from every other one. Further- 

 more, from its earliest to its latest stage of development it is one 

 and the same organism ; the egg is not one being and the embryo 

 another and the adult a third, but the egg of a human being is a 

 human being in the one-celled stage of development, and the char- 

 acteristics of the adult develop out of the egg and are not in some 

 mysterious way grafted upon it or transmitted to it. 



Parents do not transmit their characters to their offspring, but 

 their germ cells in the course of long development give rise to 

 adult characters similar to those of the parents. The thing which 

 persists more or less completely from generation to generation 

 is the organization of the germ cells which differentiate in similar 

 ways in successive generations if the extrinsic factors of develop- 

 ment remain similar. 



