Chap. I.] INTRODUCTORY. 



addition of new material, but by the incorporation of that new 

 material into the very substance of the old ; and (3) that the 

 material incorporated during growth differs from the material 

 absorbed from without, which has thus undergone a chemical 

 transformation within the animal. The growth of the organism 

 is dependent upon the continued absorption of new material 

 from without, and its transformation into the substance of the 

 body. 



But, after a while, the growth of the living animal ceases. It 

 is, as we say, fully grown. Why, then, continue the process of 

 intus-susception, as it is called, that is, the incorporation of new 

 material from without ? Because, if it be not continued, the 

 animal wastes away and dies. And thus a new fact comes to 

 light, that of constant waste, which must be made good by con- 

 stant repair. So that we may say that a living animal is a centre 

 of continual waste and repair, of nicely-balanced constructive and 

 destructive processes. 



Only so long as the constructive processes outbalance the 

 destructive processes does growth continue. During the greater 

 part of a healthy man's life, for example, the two processes, 

 waste and repair, are in equilibrium. In old age waste slowly 

 but surely gains the mastery ; and, at death, it sets in unchecked 

 by repair. 



So far, then, a living animal is a centre of continual waste and \ 

 repair, which may or may not be accompanied by growth. Let / 

 us now look at this growth a little more closely. There is some- 

 thing more than growth in the passage of the infant into the 

 man. There is development as well. But take a more marked 

 case. In spring and early summer there is plenty of frog-spawn 

 in the ponds. A number of blackish specks of the size of 

 mustard seeds are imbedded in a jelly-like mass. They are 

 frogs' eggs. They seem unorganised. But watch them, and the 

 organisation will gradually appear. The egg will be hatched, 

 and give rise to a little fish-like creature (see Fig. 2, p. 3). This 

 will gradually grow into a tadpole, with a powerful swimming 

 tail. Legs will appear. The tail will shrink in size and be 

 drawn into the body. The tadpole will have developed into a 



