Ouap. VIII. Sexual Selection. 237 



10 one sex. whilst others are common to both sexes. A difference 

 of this kind in the period of development is not so improbable as 

 it may at first appear ; for with the Orthoptera, which assume 

 their adult state, not by a single metamorphosis, but by a suc- 

 cession of moults, the young males of some species at first 

 resemble the females, and acquire their distinctive masculine 

 characters only at a later moult. Strictly analogous cases occur 

 at the successive moults of certain male crustaceans. 



We have as yet considered the transference of characters, re- 

 latively to their period of development, only in species in a 

 natural state; we will now turn to domesticated animals, and 

 first touch on monstrosities and diseases. The presence of super- 

 numerary digits, and the absence of certain phalanges, must be 

 determined at an early embryonic period— the tendency to profuse 

 bleeding is at least congenital, as is probably colour-blindness — 

 yet these peculiarities, and other similar ones, are often limited 

 in their transmission to one sex ; so that the rule that 

 characters, developed at an early period, tend to be trans- 

 mitted to both sexes, here wholly fails. But this rule as 

 before remarked, does not appear to be nearly so general as the 

 converse one, namely, that characters which appear late in life 

 in one sex are transmitted exclusively to the same sex. From 

 the fact of the above abnormal peculiarities becoming attached 

 to one sex, long before the sexual functions are active, we may 

 infer that there must be some difference between the sexes at an 

 extremely early age. With respect to sexually-limited diseases, 

 we know too little of the period at which they originate, to draw 

 any safe conclusion. Gout, however, seems to fall under our 

 rule, for it is generally caused by intemperance during manhood, 

 and is transmitted from the father to his sons in a much more 

 marked manner than to his daughters. 



In the various domestic breeds of sheep, goats, and cattle, the 

 males differ from their respective females in the shape or develop- 

 ment of their horns, forehead, mane, dewlap, tail, and hump on 

 the shoulders ; and these peculiarities, in accordance with our 

 rule, are not fully developed until a rather late period of life. 

 The sexes of dogs do not differ, except that in certain breeds, 

 especially in the Scotch deer-hound, the male is much larger 

 and heavier than the female ; and, as we shall see in a future 

 chapter, the male goes on increasing in size to an unusually late 

 period of life, which, according to rule, will account for his in- 

 creased size being transmitted to his male offspring alone. On 

 the other hand, the tortoise-shell colour, which is confined to 

 female cats, is quite distinct at birth, and this case violates the 

 lule. There is a breed of pigeons in which the males alone are 



