PRESENT CRISIS IN EVOLUTIONARY THOUGHT 25 



cytological phenomena of synapsis, meiosis, and syngamy, 

 has explained for us the instability of hybrids, has placed 

 Weismann's speculations concerning the autonomy and con- 

 tinuity of the germ, plasm on a firm basis of experimental 

 fact, has clarified all our notions respecting the mode and 

 range of hereditary transmission, and has, in a word, opened 

 our eyes to that new and hitherto unexplored realm of nature 

 which Bateson calls ''the world of gametes." 



Efforts have been made to construct systems of transform- 

 ism along Mendelian lines, but none of them has met with 

 notable success. Lotsy, for example, sought to explain all 

 variation on the basis of the rearrangement of preexistent 

 genetic factors brought about by crossing. But such a solution 

 of the problem is very unsatisfactory. In the first place, the 

 generality of hybrid (heterozygous) forms are ruled out on the 

 score of instability. The phenotype of hybrids is directly 

 dependent, not on the genes themselves, but on the diploid com- 

 bination of genes contained in the zygote. This combination, 

 however, is always dissolved in the process of gamete-forma- 

 tion, by the segregative reduction division which occurs in the 

 reproductive organs of the hybrid. Hybrids, therefore, do not 

 breec? trv£, if propagated by sexual reproduction. To maintain 

 constancy of type in hybrids, one must resort to somatogenic 

 reproduction {i.e. vegetative growth from stems, etc.). Certain 

 violets, in fact, as well as blackberries, are maintained in a 

 state of constant hybridism by means of this sort of reproduc- 

 tion, even in nature. In the case of balanced lethals {i.e. fac- 

 tors causing death in the pure Qr homozygous state), the 

 hybrid phenotype may be maintained even by sexual re- 

 production, inasmuch as all the pure (homozygous) off- 

 spring are non-viable. Two lethals are said to be balanced, 

 when they occur, the first in one and the second in the 

 other homologous chromosome of the same synaptic pair. 

 "Such a factorial situation would maintain a state of constant 

 heterozygosis, the fixed hybridism of an impure species . . . 

 the hybrid will breed true until the relative position of the 



