234 THE DESCENT OF MAN. 



nacity, are now generally exhibited in separate pens. With the 

 Polish breeds the bony protuberance of the skull which supports 

 the crest is partially developed even before the chickens are 

 hatched, and the crest itself soon begins to grow, though at first 

 feebly;" and in this breed the adults of both sexes are charac- 

 terized by a great bony protuberance and an immense crest. 



Finally, from what we have now seen of the relation which 

 exists in many natural species and domesticated races, between 

 the period of the development of their characters and the manner 

 of their transmission — for example, the striking fact of the early 

 growth of the horns in the reindeer, in which both sexes bear 

 horns, in comparison with their much later growth in the other 

 species in which the male alone bears horns — we may conclude 

 that one, though not the sole cause of characters being exclusively 

 inherited by one sex, is their development at a late age. And 

 secondly that one, though apparently a less efficient cause of char- 

 acters being inherited by both sexes, is their development at an 

 early age, whilst the sexes differ but little in constitution. It 

 appears, however, that some difference must exist between the 

 sexes even during a very early embryonic period, for characters 

 developed at this age not rarely become attached to one sex. 



Summary and concluding remarlis. — From the foregoing dis- 

 cussion on the various laws of inheritance, we learn that the 

 characters of the parents often, or even generally, tend to become 

 developed in the offspring of the same sex, at the same age, and 

 periodically at the same season of the year, in which they first 

 appeared in the parents. But these rules, owing to unknown 

 causes, are far from being fixed. Hence during the modification 

 of a species, the successive changes may readily be transmitted 

 in different ways; some to one sex, and some to both; some to 

 the offspring at one age, and some to the offspring at all ages. 

 Not only are the laws of inheritance extremely complex, but so 

 are the causes which induce and govern variability. The varia- 

 tions thus induced are preserved and accumulated by sexual selec- 

 tion, which is in itself an extremely complex affair, depending, as 

 it does, on the ardor in love, the courage, and the rivalry of the 

 males, as well as on the powers of perception, the taste, and will 

 of the female. Sexual selection will also be largely dominated by 

 natural selection tending towards the general welfare of the spe- 

 cies. Hence the manner in which the individuals of either or both 



*' For full particulars and references on all these points respecting 

 the several breeds of the Fowl, see 'Variation of Animals and Plants 

 under Domestication,' vol. i. pp. 250, 256. In regard to the higher ani- 

 mals, the sexual differences which have arisen under domestication 

 are described in the same work under the head of each species. 



