10 THE ORIGIN OF SPECIES 



volumes, is the fullest and the best on this subject. No breeder 

 doubts how strong is the tendency to inheritance; that like pro- 

 duces like, is his fundamental belief; doubts have been thrown on 

 this principle only by theoretical writers. When any deviation of 

 structure often appears, and we see it in the father and child, we 

 cannot tell whether it may not be due to the same cause having 

 acted on both ; but when among individuals, apparently exposed to 

 the same conditions, any very rare deviation, due to some ex- 

 traordinary combination of circumstances, appears in the parent 

 — say, once among several million individuals — and it reappears in 

 the child, the mere doctrine of chances almost compels us to at- 

 tribute its reappearance to inheritance. Every one must have heard 

 of cases of albinism, prickly skin, hairy bodies, etc., appearing in 

 several members of the same family. If strange and rare deviations 

 of structure are really inherited, less strange and commoner devi- 

 ations may be freely admitted to be inheritable. Perhaps the cor- 

 rect way of viewing the whole subject would be, to look at the 

 inheritance of every character whatever as the rule, and non- 

 inheritance as the anomaly. 



The laws governing inheritance are for the most part unknown. 

 No one can say why the same peculiarity in different individuals 

 of the same species, or in different species, is sometimes inherited 

 and sometimes not so ; why the child often reverts in certain char- 

 acteristics to its grandfather or grandmother or more remote an- 

 cestor; why a peculiarity is often transmitted from one sex to 

 both sexes, or to one sex alone, more commonly but not exclusively 

 to the like sex. It is a fact of some importance to us, that peculiari- 

 ties appearing in the males of our domestic breeds are often trans- 

 mitted, either exclusively or in a much greater degree, to the males 

 alone. A much more important rule, which I think may be trusted, 

 is that, at whatever period of life a peculiarity first appears, it 

 tends to reappear in the offspring at a corresponding age, though 

 sometimes earlier. In many cases this could not be otherwise: thus 

 the inherited peculiarities in the horns of cattle could appear only 

 in the offspring when nearly mature; peculiarities in the silkworm 

 are known to appear at the corresponding caterpillar or cocoon 

 stage. But hereditary diseases and some other facts make me be- 

 lieve that the rule has a wider extension, and that, when there is 

 no apparent reason why a peculiarity should appear at any par- 

 ticular age, yet that it does tend to appear in the offspring at the 

 same period at which it first appeared in the parent. I believe this 

 rule to be of the highest importance in explaining the laws of em- 

 bryology. These remarks are of course confined to the first appear- 



