

180 HYBRIDISM. 



Chap. XIX 



constitutional differences, for annual and perennial plants, 

 deciduous and evergreen trees, plants flowering at different 

 seasons, inhabiting different stations, and naturally living under 

 the most opposite climates, can often be crossed with ease. 

 The difficulty or facility apparently depends exclusively on the 

 sexual constitution of the species which are crossed ; or on their 

 sexual elective affinity, i e. Wahlverwandtschaft of Gartner. As 

 species rarely or never become modified in one character, 



without being at the same time modified in many, and as sys- 



tematic affinity includes all visible resemblances and dissimi- 

 larities, any difference in sexual constitution between two species 

 would naturally stand in more or less close relation with their 

 systematic position. 



Sixthly, the sterility of species when first crossed, and that 

 of hybrids, may possibly depend to a certain extent on distinct 

 causes. With pure species the reproductive organs are in a 

 perfect condition, whilst with hybrids they are often plainly 

 deteriorated. A hybrid embryo which partakes of the constitu- 

 tion of its father and mother is exposed to unnatural conditions, 

 as long as it is nourished within the womb, or egg, or seed of 

 the mother-form : and as we ' know that unnatural conditions 



often induce sterility, the reproductive organs of the hybrid might 



at this early age be permanently affected. But this cause has 

 no bearing on the infertility of first unions. The diminished 

 number of the offspring from first unions may often result, as is 

 certainly sometimes the case, from the premature death of most 

 of the hybrid embryos. But we shall immediately see that a 

 law of an unknown nature apparently exists, which causes the 

 offspring from unions, which are infertile, to be themselves more 

 or less infertile ; and this at present is all that can be said. 



Seventhly, hybrids and mongrels present, with the one great 

 exception of fertility, the most striking accordance in all other 



r 



ipects ; namely, in the laws of their resemblance to their 



parents, in their tendency to reversion, in their variability, and 



being absorbed through repeated crosses by either pai 



for 



m 



Since arriving at the foregoing conclusions, condensed from 

 my former work, I have been led to investigate a subject which 

 throws considerable light on hybridism, namely, the fertility ot 



