346 



The Joi'RNAL OF Heredity 



nature of individual diflfcrcnccs among 

 the members of nonnally diverse species, 

 with free intercrossing of different Hnes 

 of descent. We are familiar with 

 diversity in the human s])ecies, and do 

 not expect to find two indi\'iduals with 

 exactly the same characters, except in 

 cases of identical twins, and here the 

 phenomenon of uniformity is ascribed 

 to a division of the fertilized egg cell, 

 analogous to vegetative propagation. 

 As far as observation has gone, a 

 similar diversity exists everywhere in 

 nature, except where it is restricted by 

 special methods of reproduction. To fit 

 the facts of nature our theories must 

 provide enough gens and in sufficiently 

 intimate relations with each other to 

 equip this infinite diversity of indi- 

 \-idual forms. 



CHARACTERS AND LINES OF DESCENT. 



Instead of trying to think of gens as 

 separate entities or corpuscles of any 

 sort we may think of them as parts of 

 the ancestral lines that form the net- 

 work of descent of the species. A better 

 idea of the nature of the germ-plasm 

 may be gained by considering it as a 

 part of this network of descent. The 

 cells that make up the bodies of plants 

 or animals are connected into networks 

 by delicate threads that pass through 

 the cell walls, and the i^rotoplasm inside 

 the cells also has a reticular structure. 

 Thus there is no lack of analogy for as- 

 suming a relation between characters 

 and lines of descent. Indeed, we have 

 all the phenomena of sexuality to war- 

 rant the suggestion of a physiological or 

 causal relation between the network of 

 the species and the network of the cell. 

 As the crossing of lines of descent 

 und(juV)tedly has a ]:)hysiological func- 

 tion in increasing and maintaining the 

 efficiency of the proto]:)lasm, those 

 who dwell on the idea of a mechanical 

 basis for the j^henomena of life should 

 be the more ready to look upon the 

 reticulum of the cell as representing 

 the network of descent of the sjjccies. 

 This relation or analogy allows us to 

 consider the exi)ression of each char- 

 acter as marking a stage in the develo])- 

 mcnt of the individual, and at the same 

 time as corresjumding to one of the 



many ancestral lines or alternative 

 courses of development that are open 

 to the members of a diverse species 

 propagating itself in a normal network 

 of descent. 



Selective line breeding, self-fertiliza- 

 tion, or vegetative ])ropagation, enable 

 us to establish definite courses through 

 the maze of alternative paths of expres- 

 sion. When regularity of expression 

 has been secured, so that all the mem- 

 bers of a stock bring the same characters 

 into expression, we have a unifijmi 

 "pure" race. Uniformity has been 

 acce]Jted in the monogenic theory of 

 Mendclism as representing the normal 

 condition of heredity, though breeders 

 know that uniformity is generally arti- 

 ficial, as well as temporary. Even in 

 the most carefully selected stocks muta- 

 tions continue to appear, the "sports," 

 "rogues" or "black sheep" that are 

 rejected because they do not follow the 

 standard course of development. Thus 

 the breeder's care is required to main- 

 tain the uniformity of a select stock, no 

 less than to originate it in the first 

 place. 



The analog}' l^ctween characters and 

 lines of descent also allows us to think 

 of successive appearances of the char- 

 acters in the development of the indi- 

 vidual as corresponding to their succes- 

 sive develoijment in the evolutionary 

 progress of the species. It is not neces- 

 sary to suppose that the embr\-ological 

 record is entirely consistent or complete. 

 New si)ecializations, such as the lar\-al 

 characters of insects, may be intercalated 

 into the early stages of the life history, 

 instead of being added at the end. The 

 extreme form of the recapitulation 

 theor\-. that the embryonic and juvenile 

 characters rei)resent the exact courses 

 of evolutionary i)rogress, has been 

 aV)and()ned, but this need not interfere 

 with th(^ recognition of a general analogy 

 or ])arallel between e\-olutionar\' de\-el- 

 ojjment, or transmission of characters, 

 and individual development, or expres- 

 sion of characters. Each character may 

 still be thought of as a stage of develop- 

 ment, reached by following one of the 

 ancestral courses or lines of descent. 



The conce])tion of characters gained 

 by this analogy is more concrete and 



