Origin of Sex. 295 



remaining attached together to form a bod} T or individual. If you take 

 a piece of grape vine or slip of currant bush and bend it down and cover 

 with a little earth, it will soon develope roots. Without disconnecting 

 this slip from its parent root, by a strictly continuous growth, it has be- 

 come a new bush or vine. But it may be cut off from its parent before 

 developing new roots, and as a mere slip be sent by mail across the conti- 

 nent, and then when stuck into the ground will develop roots and grow 

 as a new plant. Here the growth is discontinuous certainly, and it is 

 reproduction, but it does not differ in any essential particular from con- 

 tinuous growth. You may cut a riding whip from a willow tree, and if, 

 after accomplishing by its help a journey of forty miles, you stick it in 

 a moist spot of earth, it will grow into a tree. But if you make the ex- 

 periment with a peach, apple or oak switch it will not work. These 

 switches will never produce roots. There has been a differentiation in 

 these trees by which the common branch has lost the power of reproduc- 

 ing the roots, and this power has become invested in a certain concen- 

 trated part of arrested growth ; viz, the peach stone, apple seed or acorn. 

 The limb or twig retains the power of the growth resulting from the 

 divisions and multiplication of its own cells, and you may take the twig 

 of the apple or the bud of the peach and stick it into another tree in 

 such a way that it can get proper nourishment, and it will continue to 

 reproduce its cells and grow. It is so of the higher animals also. There 

 has been such a differentiation that, while all the parts continue to grow 

 by the asexual reproductive process of the cells of their own tissues, the 

 possibility of reproducing all the tissues in a complete animal is not re- 

 tained in every cell, but is transferred to certain specialized reproductive 

 cells. In a Star-fish (as in a Willow ) , one limb or ray detached will re- 

 produce the other four and re-form the animal complete. In an Amphib- 

 ian the differentiation has proceeded so far that the limb can no longer 

 reproduce the rest of the animal, but the rest of the animal can repro- 

 duce it, so that a salamander may have a new tail or a frog a new leg. 

 In the mammal the differentiation has become still more pronounced, 

 so much so that it is rare that a lost limb grows on again. But the 

 power of asexual cell reproduction is still retained by all the separate 

 tissues. If it were not so there could be no growth except of excretory 

 matter. This power enables the tissues to repair themselves ( within 

 certain limits ) like a broken crystal. A finger nail torn off, or hair 

 plucked out, or a bit of skin stript off will be replaced ; a -muscle, nerve, 

 blood-vessel, or bone disrupted will be knit together again. Thus, as 

 we advance up the scale of organization, we find the differentiation of 

 the tissues to become so pronounced that the cells of one tissue, while 

 they reproduce their own kind, cannot reproduce those of another tissue, 

 and the highly organized body may be likened remotely to a conglom- 



