Habit. 315 



It was observed above that among the higher hermaphrodites self im- 

 pregnation is not usual, and in many cases it is not possible, for reasons 

 stated. Amongst the higher mammals, including man, breeding be- 

 tween individuals nearly related by blood, is often not possible, and 

 when it does happen it is apt to result in a deformed progeny. Sexual 

 unions of this sort constitute a return towards the conditions of her- 

 maphroditism. True hermaphrodites, that is, individuals possessing 

 both kinds of sexual glands in working order, are not met with amongst 

 the higher mammals, or mankind, although partial hermaphrodites, or 

 those having both sorts in various degrees of development short of func- 

 tional perfection, are occasionally found, and are to be considered as 

 cases of arrested development. But a male and a female just alike in 

 aU respects, except that one possesses the male and the other the fe- 

 male glands, considered together constitute the equivalent of a true her- 

 maphrodite, because the latter is, to all intents and purposes, such a 

 pair consolidated into one or rather such a pair not } r et separated into 

 two. A pair nearly related by blood is apt to, more or less complete^, 

 fill these conditions ; the reproductive elements of such a pair are not 

 complemental and do not supply each other's deficiencies; what one lacks 

 they both lack. 



CHAPTER XXXIY. 



HABIT. 



If we now pause to take a brief retrospect of the ground thus far 

 passed over we shall find Habit appearing as the immediate and visible 

 cause of almost every organic change, whether in development or evolu- 

 tion. In the development of the embryo the successive changes follow 

 each other in the order which the habits of myriads of generations have 

 established. In the advance by way of these habitual changes the em- 

 bryo is carried through numerous forms of no permanent use to it ; 

 forms which are maintained for only a brief space and then modified 

 and altered out of recognition. No rational account of such an indirect 

 and wasteful mode of developmental progress can be given, except that 

 nature possessed no cheaper way, in general, than the way of habit. 

 Wherever new habits have been introduced, such introduction has been 

 effected by extremely slow processes, which have invariably consisted in 

 the gradual modification of old habits a modification so gradual, and 

 taken on by steps so infinitessimal that every habit is, to a greater or 

 less extent, always an old habit. What has been said on bilateralism, 

 on the evolution of the osseous s} T stem, on the effects of food, periodic- 

 ity, temperature, water and dessication, on respiration and parasitism, 



