ZOOGENESIS 



organs of the body and forming either of themselves 

 or as a result of their activity the diverse struc- 

 tural elements. 



Since every animal, no matter what it is, originates 

 as a single cell, and the body of every animal is com- 

 posed of one or many cells which are always essen- 

 tially the same in structure, we clearly see that all 

 types of animal life must be explained in terms of a 

 single cell. 



Unbroken continuity of life from its very first 

 appearance on the earth, and the fact that all animals 

 begin as single cells and no matter what their size 

 are always composed of a single cell or of a multitude 

 of cells which in their broader features resemble the 

 original single cells from which they are derived — 

 these are the fundamentals out of which we must 

 construct our picture of the interrelationships of ani- 

 mal forms. 



Our first reaction to the plain statement of these 

 fundamentals is that the problem is a simple one. 

 Since all animals begin as a single cell, and since many 

 animals are known whose entire body consists of a 

 single cell, therefore these single celled animals must 

 be the most primitive and all other animals must have 

 been descended from them. So we are tempted to 

 construct an evolutionary line from the single celled 

 animals to those whose body is most complex in 

 terms of cells, and then to arrange all animal types as 

 best we may along this line. Such an assumed course 

 of development of animal forms, from those whose 

 body is composed of a single cell to the multitudes of 



