164 HYBRIDISM. Chap. XIX. 



longing to distinct genera can rarely, and those belonging 

 to distinct families can never, be crossed. The parallelism, 

 however, is far from complete ; for a multitude of closely 

 allied sjiecies will not unite, or unite with extreme difficulty, 

 whilst other species, widely different from one another, can 

 be crossed with perfect facility. Nor does the difficulty 

 depend on ordinary 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 ap- 

 parently 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 characters, and as systematic 

 affinity includes all visible similarities and dissimilarities, 

 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 con- 

 stitution 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 niay 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 leads to the offspring from 

 unions, which are infertile, being themselves more or less 

 nfertile ; and this at present is all that can be said. 

 Seventhly, hybrids and mongrels present, with the one great 





