264 FOUNDATIONS OF BIOLOGY 



animals which are forthcoming as the principles already 

 known are applied, and subsidiary ones are discovered. And 

 last but not least, Man has begun to study himself as a prod- 

 uct of breeding and the process of evolution to determine 

 the distribution of characters in the family, and the conse- 

 quences of their combinations in the physical and mental 

 make-up of the individual. 



A. HERITABILITY OF VARIATIONS 



What then are the basic principles of heredity which are 

 to-day at the command of the scientific breeder? To answer 

 this question it is necessary to go into some details because 

 no real appreciation of the underlying principles involved is 

 otherwise forthcoming. Most of these details have been ac- 

 quired through patient investigations made from the standpoint 

 of so-called pure science one more proof of the indebtedness 

 of the ' practical man of affairs ' to the biological laboratory. 



In the Protista the problems of heredity confront us 

 in their simplest, though by no means simple, form. Para- 

 mecium, as we know, divides into two cells which through 

 growth and reorganization soon are to all intents and 

 purposes replicas of the parent cell. The parent has merged 

 its individuality into that of its offspring. Thus stated, one 

 does not wonder that parent and offspring are alike each 

 is composed of essentially the same protoplasm. But when 

 we come to multicellular forms in which reproduction is 

 restricted to special germ cells which involve fertilization, 

 confusion is apt to arise unless one keeps clearly in mind 

 and perhaps exaggerates for the sake of concreteness the 

 distinction between germ and soma which has been 

 previously discussed. Since in higher forms, to which brevity 

 demands that our attention be confined, the sole connection 

 between parent and offspring is through the germ cells, it 



