26 THE CASE AGAINST EVOLUTION 



lethals are changed by a crossover, or the genetical constitu- 

 tion in these respects is altered by a mutation." (Davis, 

 Science, Feb. 3, 1922, p. 111.) As is evident, however, the 

 condition of balanced lethals involves a considerable reduc- 

 tion in fertility. 



Hybridization, moreover, is successful between varieties of 

 the same species rather than between distinct species. Inter- 

 specific crosses are in some cases entirely unproductive, in 

 other cases productive of wholly-sterile, hybrids, and in still 

 other cases productive of semisterile hybrids. When semi- 

 sterile hybrids are obtainable from an interspecific cross, the 

 phenotype can be kept constant by somatogenic reproduction, 

 but, as we shall see in a later chapter, this kind of reproduction 

 does not counteract senescence, and stock thus propagated 

 usually plays out within a determinate period. Finally, the 

 mixture of incompatible germinal elements involved in an 

 interspecific cross tends to produce forms, which are subnormal 

 in their viability and vitality. The conclusions of Goodspeed 

 and Clausen are the following: "(1) As a consequence of 

 modern Mendelian developments, the Mendelian factors may 

 be considered as making up a reaction system, the elements 

 of which exhibit more or less specific relations to one another ; 

 (2) strictly Mendelian results are to be expected only when 

 the contrast is between factor differences within a common 

 Mendelian reaction system as is ordinarily the case in varie- 

 tal hybrids; (3) when distinct reaction systems are involved, 

 as in species crosses, the phenomena must be viewed in the 

 light of a contrast between systems rather than between spe- 

 cific factor differences, and the results will depend upon the 

 degree of mutual compatibility displayed between the specific 

 elements of the two systems." (Amer. Nat., 51 (1917), p. 99.) 

 To these conclusions may be added the pertinent observation 

 of Bradley Moore Davis: "Of particular import," he says, "is 

 the expectation that lethals most frequently owe their presence 

 to the heterozygous condition since the mixing of diverse germ 

 plasms seems likely to lead to the breaking down of delicate 



