EVOLUTION AND ANIMAL LIFE 



species the operation of world-wide forces, the cooperation of 

 great influences, far-reaching in time and space, as broad as the 

 surface of the globe and as enduring as its Hfe. To consider 

 these causes so far as known is the purpose of this work. Our 

 problem is no longer the "mystery of mysteries,^' for in a large 

 way by the work of Darwin and his successors the influences 

 promoting variety in life are already known. We know many 

 of the different factors which produce divergence in form and 

 adaptation to conditions. But the relative value of these 

 factors is less certain, and from time to time other and more 



Fig. 12. — Lizard walking. (After Marey.) 



subtle factors are brought to light, or the great forces them- 

 selves are analyzed into finer component elements. 



But with all that we may say of the universality of variation 

 and the prevalence of individualism, we are equally impressed 

 with the underlying unity. There are onl}^ a few types of 

 structure among animals, and in these few the beginnings in 

 development are the same. The plants show similarly a few 

 modes of development, and all the range of families and forms 

 is based on the modification of a few simple types. Moreover 

 all living forms, plants and animals alike, agree in the funda- 

 mental elements. All are made of a framework of cells, each 

 cell a source of energy, containing in all cases a semifluid net- 

 work of protoplasm, which is found wherever the phenomena 

 of life appear. In all the cells is the mysterious nuclear sub- 

 stance which seems to direct the operations of heredity. The 

 same laws or methods of heredity, variability, and response to 



