232 PLANT HYBRIDIZATION BEFORE MENDEL 



Darwin frequently emphasizes the same view regarding the 

 differentiating effects of a new environment. 



"But hardly any cases afford more striking evidence how powerfully 

 a change in the conditions of life acts on the sexual elements, than those 

 already given, of plants which are completely self-sterile in one country, 

 and when brought to another, yield, even in the first generation, a fair 

 supply of self-fertilized seeds." (lb, p. 452,) And again, ". . . We know 

 that a plant propagated for some generations in another garden in the 

 same district serves as a fresh stock and has high fertilizing powers. 

 The curious cases of plants which can fertilize and be fertilized by any 

 other individual of the same species, but are altogether sterile with 

 their own pollen, become intelligible, if the view here propounded is 

 correct, namely, that the individuals of the same species growing in a 

 state of nature near together have not really been subjected during sev- 

 eral previous generations to quite the same conditions." (lb, pp. 455-6.) 



"when two varieties which present well-marked differences are crossed, 

 their descendants in the later generations differ greatly from one another 

 in external characters; and this is due to the augmentation or oblitera- 

 tion of some of these characters, and to the reappearance of former ones 

 through reversion ; and so it will be, as we may feel almost sure, with 

 any slight differences in the constitution of their sexual elements." (lb, 

 p. 454.) 



With regard to the ill effects derived from self-fertilization, 

 Darwin says : 



"whether with plants the evil from self-fertilization goes on increas- 

 ing during successive generations is not as yet known ; but we may infer 

 from my experiments that the increase, if any, is far from rapid. After 

 plants have been propagated by self-fertilization for several genera- 

 tions, a single cross with a fresh stock restores their pristine vigour, 

 and we have a strictly analogous result with the domestic animals. The 

 good effects of cross-fertilization are transmitted by plants to the next 

 generation; and, judging from the varieties of the common pea, to many 

 succeeding generations. But this may merely be that crossed plants of 

 the first generation are extremely vigorous, and transmit their vigour, 

 like any other character, to their successors." (lb, p. 444.) 



In this paragraph, Darwin calls attention to a fact already 

 referred to, that attracted little attention for a generation, viz., 

 the immediate improvement due to a cross, known as "heterosis." 

 Darwin was thus, if not the first to call sharply to attention the 

 matter of the relatively increased size and vigor of first genera- 

 tion hybrids, at least the first to subject the question to experi- 

 mental analysis. 



Darwin supposed that what occurred in the case of hybridiza- 

 tion was a general breaking-up of the plant's characters, hybrid- 

 ization being understood to operate in about the same wav upon 

 the plant's organization as do changes in the external conditions. 



