166 DARWINISM chap. 



Parallelism between Crossing and Change of Conditions. 



In the Avliole series of these phenomena, from the beneficial 

 effects of the crossing of different stocks and the evil effects of 

 close interbreeding, up to the partial or complete sterility 

 induced by crosses between species belonging to different 

 genera, we have, as Mr. Darwin points out, a curious parallelism 

 with the effects produced by change of physical conditions. 

 It is well known that slight changes in the conditions of life 

 are beneficial to all living things. Plants, if constantly grown 

 in one soil and locality from their own seeds, are greatly 

 l>enefited by the importation of seed from some other locality. 

 The same thing happens with animals ; and the benefit we our- 

 selves experience from " change of air " is an illustration of 

 the same phenomenon. But the amount of the change which 

 is beneficial has its limits, and then a greater amount is 

 injurious. A change to a climate a few degrees warmer or 

 colder may be good, while a change to the tropics or to the 

 arctic regions might be injurious. 



Thus we see that, both slight changes of conditions and 

 a slight amount of crossing, are beneficial ; while extreme 

 changes, and crosses between individuals too far removed in 

 structure or constitution, are injurious. And there is not 

 only a parallelism but an actual connection between the two 

 classes of facts, for, as we have already shown, many species 

 of animals and plants are rendered infertile, or altogether 

 sterile, by the change from their natural conditions which 

 occurs in confinement or in cultivation ; while, on the other 

 hand, the increased vigour or fertility which is invariably pro- 

 duced by a' judicious cross may be also effected by a judicious 

 change of climate and surroundings. We shall see in a subse- 

 quent chapter, that this interchangeabilityof the beneficial effects 

 of crossing and of new conditions, serves to explain some very 

 puzzling phenomena in the forms and economy of flowers, 



Eemarks on the Facts of Hyhridity. 



The facts that have now been adduced, though not very 

 numerous, are sufficiently conclusive to prove that the old 

 belief, of the universal sterility of hybrids and fertility of 

 mongrels, is incorrect. The doctrine that such a universal 



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