296 Dynamic Theory. 



erate rock composed of a number of different sorts of crystals, each in- 

 dependent of the rest and yet necessary to the integrity of the whole. 

 The wider the tissues are differentiated from each other, the further are 

 their constituent cells from the condition of those original cells of universal 

 function with which the power of the reproduction of all the tissues al- 

 ways remains ; and the narrower the limits within which the tissue cells 

 may vary from their normal quality. This being so, I think it neces- 

 sarily follows that the functions remaining in the reproductive cells are 

 likewise intensified, and that therefore the higher organisms breed truer 

 than the lower ones, an inference which I believe observation amply 

 bears out. The reproductive cells themselves, like the cells which form 

 the tissues, are subject to growth and division, which in their case, con- 

 sidered as cells simply, constitutes their asexual reproduction. 



Asexual reproduction, which is the rule for the growth, continuous or 

 discontinuous of the simplest animals, and of the simple tissues of the 

 complex animals and plants, is superseded for the reproduction of the 

 body in all the more highly differentiated organisms by sexual reproduc- 

 tion. Sexuality is a condition of things in which the cells differentiated 

 to reproduction have undergone a further differentiation, so that one- 

 half of them have lost one part of the reproductive function which has 

 been assumed by the other half, in doing which this latter half has lost 

 another part of the reproductive function, which has in like manner 

 been assumed by the first half ; so that the general reproduction of the 

 organism cannot proceed till two cells having these complemental frac- 

 tions of function be got together. This differentiation has been a 

 gradual process, and we find it in various degrees of emphasis and com- 

 pleteness. In those asexual animals which are only so far differen- 

 tiated as to have the reproductive cells distinct from those producing 

 tissues, the reproductive cells are usually to be found together in one 

 place and developing in connection with a special gland. And when sex 

 differentiation begins, both kinds of sex cells are at first found develop- 

 ing in the same gland. Then the two kinds are developed alternately ; 

 then one kind at one period in the life of the individual, and the other 

 kind at another time from the same gland. Then there are two glands, 

 one for the development and keeping of the female cells, or ova, and 

 the other for the male cells, or spermatozoa. Both these glands are at 

 first in one individual and near together ; later they become separated in 

 different parts of the same animal; and finally the female cells are found 

 in one individual and the male cells in another. When both kinds are 

 found in a single individual, it is called an hermaphrodite ; when an in- 

 dividual has but one kind it is unisexual. Amongst both plants and 

 animals are to be found these three conditions as regards reproduction ; 

 viz., asexual reproduction, hermaphroditism, unisexuality. Most plants 



