208 LEISURE-TIME STUDIES. 



from the original form. A nai's cut into forty pieces was 

 transformed through the operation into as many small worms 

 of its own kind. Here the law of likeness or heredity 

 operates in the plainest and most direct fashion. The young 

 are like the parent-stock, because they consist in reality of 

 detached portions of the parent's personality. The experi- 

 ments of naturalists carried out on animals of lower organisa- 

 tion than these worms, such as the little fresh-water polype 

 or hydra,* show a power of artificial reproduction which is of 

 literally marvellous extent ; and all such animals evince at 

 once the simplest mode of development and the plainest 

 reasons why the young should exactly resemble the parent. 

 It might, however, be alleged that such artificial experimen- 

 tation was hardly to be accepted as illustrative of natural de- 

 velopment ; but in answer to such an observation the naturalist 

 might show that an exactly similar method of reproduction 

 occurs spontaneously and naturally in the nai's and in certain 

 other animals of its class. A single nai's (Fig. 27) has been 

 observed to consist of four connected but distinct portions, 

 the hinder three of which had become almost completely 

 separated from the original body, represented by the front 

 segment. A new head, eyes, and appendages could be 

 traced in course of formation upon the front extremity of 

 each of the new segments ; and as development terminated, 

 each portion could be seen to gradually detach itself from 

 its neighbours ; the original worm thus resolving itself into 

 four new individuals. The most curious feature regarding 

 this method of development consists in the fact that the 

 bodies of these worms and of nearly related animals grow 

 by new joints being added between the originally formed 

 segments and the tail. If, therefore, we suppose that one 

 of these new joints occasionally develops into a head, we 

 can form an idea of the manner in which a process, origin- 

 ally intended to increase the growth of one and a single 

 worm, becomes competent to evolve new individuals, each 

 of which essentially resembles the parent in all particulars. 

 * See page 62. 



