INTRODUCTION 



The succession of generations and the repetition of the life cycle 

 in the individual are the two great facts about which biological 

 thought centers. The problems of reproduction, growth, develop- 

 ment, inheritance, and evolution, as well as many other special 

 problems, are concerned with one aspect or another of these funda- 

 mental characteristics of life. Life as we know it exists only in the 

 form of individuals of various degrees and kinds which pass through 

 a definite series of changes and give rise to other individuals, and 

 these in turn repeat the process more or less exactly. 



At the beginning of a new generation the size of the organism is 

 usually only a fraction of that to which it finally attains. Various 

 substances are taken up by the organism as food and transformed 

 in part into the energy of its activity and in part into the material 

 substratum in which the dynamic activities occur. Under the 

 usual conditions this material substratum which constitutes the 

 visible organism increases in amount or grows during a large part 

 of the life of the organism. 



In all except perhaps the very simplest organisms another series 

 of changes occurs which we call morphogenesis or differentiation. 

 Localized differences in constitution, form, or structure appear, 

 and we say that the organism undergoes differentiation. Under 

 natural conditions this process of differentiation is very commonly 

 associated with growth, but the fact that it may occur in the com- 

 plete absence of growth shows that the association is by no means 

 a necessary one. 



Sooner or later and in one way or another the organism gives 

 rise to one or more new organisms, which like their parent are at 

 first relatively small and simple, and like it also undergo a process 

 of growth and differentiation. This is reproduction. In some of 

 the simpler forms of reproduction the parent organism divides 

 into two or more parts which constitute the new generation, and 

 there is nothing which corresponds to death in the usual sense. 

 The old individuality is replaced by new individualities, but nothing 



