286 D. F. JONES. 



significance of such a process is not to be looked for in the bringing 

 together of differentiated germplasms. 



The advantage derived from cross-fertilization is now under- 

 stood as a phenomenon of inheritance and the older hypothesis 

 of the stimulation of heterozygosis is no longer needed. Accord- 

 ing to present theory homozygous factor combinations are more 

 efficient than heterozygous combinations of the same factors. 

 In the lowest organisms which are illustrative of a primitive 

 sexuality there is direct proof that the union of different indi- 

 viduals does not result in an increased developmental efficiency. 

 Jennings (1913) finds that in Paramecium, in the generations 

 immediately following conjugation, there is a slowing down in the 

 rate of division. The advantage derived from the pairing of 

 individuals is the greater elasticity in adaptiveness resulting 

 from the mixing of different germ plasms giving to some of the 

 descendants a greater chance for survival. Pearl (1907) has 

 found that in this same organism there is a tendency for like 

 forms to conjugate due to mechanical hindrances to the pairing 

 of individuals dissimilar in form. This has been substantiated 

 by Jennings (1911). Also in gastropods Crozier (1918) has 

 demonstrated that assortative mating takes place between indi- 

 viduals of the same size and this, he considers, results in a greater 

 number of offspring than there would be if random pairing was 

 the rule. It has been proven by Pearson and Lee (1903) that 

 assortative mating occurs in man. This conclusion is reached 

 after extensive investigation in the inheritance of physical char- 

 acters in which they have found that there are positive correla- 

 tions between husband and wife with respect to stature, span of 

 arms, and length of forearm. Moreover they have shown that 

 homogamy is a factor favorable to fertility. Parents with like 

 characters are more productive of offspring. This is an impor- 

 tant observation and supports the main thesis of this paper. 



The occurrence of homogamy in such widely diverse forms of 

 life as the higher plants, protozoa, mollusca and man cannot fail 

 to have significance. The importance of discriminate isolation 

 has long been recognized since it supplies a necessary factor in 

 divergent evolution. The existence of such an assortative action 

 can now be looked for in all forms of life since evidence has been 



