EVOLUTION DEFINED 3 



protoplasm, or at least without traceable lineaments of the 

 future embryo. It was a single cell, apparently essentially like 

 any other cell, a single one of the units of structure of which 

 living organisms are made. 



Thence arose the theory of upbuilding or epigenesis (eTri, 

 upon, yeVeo-is, birth) of organisms, by the addition of cell upon 

 cell, to the original germ or egg. Each egg cell by segmentation 

 divides into two daughter cells, and these, through the influence 

 of heredity, naturally arrange themselves so that a new organ- 

 ism is formed similar to the parent organism. It was recognized 

 that the form was predetermined by the ancestry, but no 

 longer that the embryo was literally released from encasement 

 within the structure of the egg. The evolution of the individual 

 is thus conceived as the realization of an hereditary tendency. 



But "hereditary tendency" is again a metaphorical ex- 

 pression. In biology, we know no "influence" or "tendency" 

 which is not localized somewhere. Any act or modification of 

 an act is a function of some particular organ. To account for 

 the likeness involved in the facts of heredity, we must expect 

 to find some form of organic mechanism. 



Such mechanism must exist within the germ cell itself, and 

 its existence as the "physical basis of heredity" is now well 

 established. In a later chapter we shall discuss the nature 

 of this physical basis, the structures within the nucleus of the 

 germ cell which control or preside over the development of the 

 individual. From our knowledge of the operation of the cell 

 in heredity we recognize the facts of epigenesis, and with these 

 a theory of individual evolution, much more subtle than the 

 old theory of encasement. 



We may therefore still imagine the maturing of the individ- 

 ual organ as a process of evolution, or unrolling, of the 

 hereditary plan as hidden in the structure of its cells. We may 

 also speak of the same process as a development. To envelop 

 is to make snug. Development is its opposite. To develop 

 is to make free or independent. 



From the evolution of the individual it is natural to extend 

 the use of the word evolution or the word development to the 

 changes which characterize the history of a species or other 

 group of animals or plants, a process which has also been called 

 transformism or transmutation. This word transmutation de- 

 scribes the process more literally than either evolution or 



