286 THE PRINCIPLES OF [Part II. 



and an immense crest characterize the adults of both 

 sexes. 



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 iheir transmission — for ex- 

 ample, the striking fact of the early growth of the horns 

 in the reindeer, in which both sexes have horns, in com- 

 parison with their much later growth in the other species 

 in which the male alone bears horns — we may conclude 

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

 being exclusively inherited by one sex, is their develop- 

 ment at a late age. And secondly, that one, though ap- 

 parently a less efficient, cause of characters being in- 

 herited by both sexes is their development at an early 

 age, while the sexes differ but little in constitution. It 

 appears, however, that some difference must exist between 

 the sexes even during an early embryonic period, for char- 

 acters developed at this age not rarely become attached 

 to one sex. 



Summary and concluding remarks. — From the fore- 

 going discussion on the various laws of inheritance, we 

 learn that characters often or even generally tend to be- 

 come developed in the same sex, at the same age, and pe- 

 riodically at the same season of the year, in which they 

 first appeared in the parents. But these laws, from un- 

 known causes, are very liable to change. Hence the suc- 

 cessive steps in the modification of a species might readily 

 be transmitted in different ways ;*some of the steps being 

 transmitted to one sex, and some to both ; some to the 

 offspring at one age, and some at all ages. Not only are 



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

 mate, the sexual differences which have arisen under domestication ars 

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



