142 



THE AMERICAN NATURALIST [Vol.L 



it follows that so far as the phenomenon of spore degen- 

 eration is concerned some sporangia (the megasporangia 

 or seeds) are of hybrid origin and others (the micro- 

 sporangia or anthers) are not. The logical absurdity of 

 this conclusion will be clear to every one. 



There seems to be no question on the basis of the well- 

 established criteria of hybridism, that many Angiosperms 

 present clear indications that their species are of hetero- 

 zygous origin. Since one of the most efficient methods of 

 inducing variability in connection with the development 

 of improved varieties of plants is hybridization, often on 

 a very large scale, it seems not unreasonable to regard 

 spontaneous hybridization in the Angiosperms (the evi- 

 dences for which are so numerous and so impressive) as 

 having an incalculably large effect on their rate of evo- 

 lution. There is, however, apparently no reason for 

 assuming a similar condition in the Grymnosperms and the 

 vascular Cryptogams. The great and indeed overwhelm- 

 ing advantage which the Angiosperms have secured in the 

 struggle for existence over the lower groups of vascular 

 plants is apparently connected in an intimate way with 

 hybridism on the one hand and the development of her- 

 baceous types (in response to progressive climatic re- 

 frigeration) on the other. If this conclusion is correct 

 we must reject the assumption of universal hybridism as 

 the sole cause of variation put forward by Lotsy as much 

 too sweeping. Small variations unquestionably character- 

 ize the Gymnosperms, and in the course of long geological 

 time have availed in the absence of competition from 

 heterozygous types, with a much greater range of vari- 

 ability and consequently a higher potentiality of evolu- 

 tion. It is obviously impossible for the homozygous 

 Conifers to make headway against the characteristically 

 heterozygous Angiosperms. The small variations of 

 homozygous stocks clearly prevailed in the earlier history 

 of our earth, while the more rapid changes which have 

 ensued in later times are correlated, so far as plants are 

 concerned, at any rate, with marked physiographic and 



