THE MECHANISM OF DEVELOPMENT 153 



Whether or not any of these theories finally hold, this signifi- 

 cant point remains, that an adequate theory of heredity must 

 account for the following facts : 



1. A single cell thrown off from the sexual parts of a mature 

 individual will, under proper conditions, produce an entirely 

 new individual in all essential respects like the parent, but in 

 minor respects different. 



2. Commonly a cell or a number of cells taken from any 

 other part of the body will wither and die, or, if growth follows, 

 only one kind of tissue develops ; but in some instances (the 

 begonia and others) the smallest bit of leaf, under favorable 

 conditions, is able to grow and produce a new plant capable of 

 bearing blossoms and seeds. 1 



3. The mechanism of cell division seems admirably adapted 

 to insuring growth without differentiation. 



4. But differentiation does take place, and in process of 

 development a great variety of different cells arise from the 

 single original germ cell. 



5. These " differentiations " take place at different but 

 proper stages, insuring orderly arrangement and, for the most 

 part, uniform results. 



6. There is always more or less variation between individuals, 

 showing that the problem of development and inheritance is 

 something else than absolute descent without change. 



We still seek, therefore, physical units with sufficiently exact 

 properties to insure the general character of development, with 

 such mutual relations as shall provide for orderly, not simultan- 

 eous development, sufficiently elastic in their constitution (or 

 combining powers) to admit of certain deviation, and each withal 

 gifted with the power of nutrition, growth, and multiplication by 

 division. Such in general are the properties of the physiological 



determinants are united into " ids," which are held to be identical with chromatin 

 granules, and these in turn are assembled into "idants," which correspond with 

 chromosomes. For a full explanation of Weismann's theory, see his Essays on 

 Heredity, chap, iv, and his Germ Plasm, chap. i. 



1 It is a significant fact that if two begonia leaves be placed on sand simultane- 

 ously, one taken from a plant just about to blossom, the other from one just past 

 the blossoming period, the plant from the former will flower first. For Weismann's 

 views, see his Essays on Heredity, chap. iv. 



