EVOLUTIONAL ZOOLOGY 22Q 



of many hitherto utterly mysterious and inexplicable phenom- 

 ena of heredity. 



Since the coming together of the gametes at the time of 

 fertilization is a chance meeting, the law of probability deter- 

 mines the combinations that occur between them; in other 

 words, it is solely a matter of chance whether two gametes 

 will unite which bear like or unlike units, with the consequent 

 difference in the inheritance of the offspring. If this be true, 

 then the ratio in which the combinations appear, when the 

 actual breeding test is made, should approach the theoretical 

 ratio demanded by the law of probability. This has been 

 shown to be the case over and over again, and with just as 

 much certainty as the throws of dice have been proved to fol- 

 low the same law. 



Conclusion. The significance of Mendelian segregation 

 for the problem of the origin of variations and, hence, of the 

 origin of species is obvious. Since the inheritance of the 

 individual organism depends upon the unit-character content 

 of the gametes from which it arises, and since the visible re- 

 sult in the organism of these combinations is determined, fur- 

 thermore, by the physiological interaction of the unit char- 

 acters, it is clear that variations may arise suddenly and dis- 

 continuously, for the combination from which the offspring 

 is formed may be an entirely different one from that which 

 produced either of its parents. 



I cannot go farther into the subject nor follow the rami- 

 fications into which Mendelism has led us in the investi- 

 gation of the phenomena of hereditary transmission. In clos- 

 ing, let me merely call your attention to a few general con- 

 siderations which proceed from the new point of view with 

 respect to the study of heredity. 



The recognition of Mendelian "segregation" an achieve- 

 ment which is second only to the establishment of the doctrine 

 of descent itself in its far-reaching importance in biological 

 science has rescued heredity from the primitive state of mys- 



