Aim and Method of Comparative Anatomy 365 



ducing sexually, does not give rise to new individuals. They are derived, 

 not from the parental animal as a whole, but entirely from the local- 

 ized germ-plasm of the parents. The germ-plasm, becoming differen- 

 tiated into eggs and sperm atozoans, produces the entire body of a new 

 individual including, along with the various other constituent organs 

 of that body, the gonads, which are simply more germ-plasm derived 

 directly from the parental germ-plasm and destined to produce in- 

 dividuals of the next generation. 



The development of "the same" adult organ "A" (Fig. 290) in all 

 individuals of successive generations results from the presence of a 

 determining factor, or complex of factors, "a" (the "determinants" of 

 Weismann; the "genes" of modern genetics) in the germ-plasm. This 

 "a" ordinarily persists unchanged from generation to generation for an 

 indefinitely great number of generations and, accordingly, the "A" 

 organs of all the numerous individuals of the many generations are not 

 only homologous but are substantially alike in all respects. If, however, 

 a change somehow occurs in the germinal "a" complex, the organ "A" 

 will be changed in one or more particulars. Experiments show that such 

 a change may be restricted to a single germ-cell, so that only one adult 

 individual among the several of that generation may show the variation 

 or mutation in organ "A." The changed germinal factor may be indi- 

 cated by ax and the modified organ by A*. Once initiated, and pro- 

 vided that the change in the organ is favorable for the individual, or at 

 least not unduly harmful, the change may be perpetuated indefinitely 

 through successive generations of descendants, if bred only among 

 themselves. Further, it appears to be possible that the original altera- 

 tion may be augmented by additional successive variations or muta- 

 tions, and that changes affecting other characteristics of the organ 

 may be added so that, in the course of time, the organ may become A" 

 and, still later, A* rb , an organ so unlike "A" that the comparative 

 anatomist might see no reason for suspecting that the two organs were 

 related in any way. If, however, he were able to trace back along the 

 continuous germ-plasm and so discover that axrb (the germinal basis 

 for organ A" b ) arose by modification of ax, which, in turn, arose by a 

 change in the ancient "a" he would be led to conclude that A xrb is the 

 homolog of "A" and of all modified "A" organs, whether of con- 

 temporary or of most ancient animals. He would conclude also that a 

 long course of evolution must have intervened between the ancient 

 "A" and the recent A xrb , and that accordingly the animals possessing 

 A irb are only very distantly related to those having "A." But the two 

 organs, however different in form and function, are "the same" by 

 virtue of their "relation of genetic descent." 



Figure 290 illustrates a specific case, the modification of the pec- 



