20 



PRINCIPLES OF ANIMAL HIOLOGY 



are made up of cells; but when one reflects that ail the things an animal is 

 or does are what the cells are or do, the idea acquires tremendous signifi- 

 cance. All growtli, all development, all life processes, all reproduction, 

 as will be pointed out in later chapters, are the results of cell activities. 

 The universal occurrence of cells in organisms is, then, a fact of funda- 

 mental import. 



Evolution. — While, on the morphological and purely observational 

 side of biology, the cell theory was being built upon firm foundations, 

 on the philosophical side new ideas were growing up which also marked 

 progress in a different direction. Most zoologists had become so well 

 acquainted with animals as we know them at the present time that they 



Fig. 11. — Jean Baptiste Lamarck, 1744 — 1829. {From Locy's Biology and Its Makers 



and Thornton's British Plants.) 



conceived the various species to be fixed and constant. They beheved 

 each species to have been created in the form in which it exists now. 

 Through a long period of biological history, however, there had not been 

 wanting daring and inquiring naturalists who saw what they believed 

 were indications that species had changed in the past, and were perhaps 

 changing oven at the present time. Vague ideas of evolution were held 

 by some philosophers as early as the Greek period; l)ut it was not until 

 the end of the eighteenth century that any well-defined theory of evolu- 

 tion arose. Buffon (1707-1788) appears to have held evolutionary 

 ideas, but was too timid to state them clearly. Erasmus Darwin (1731- 

 1802) published in his Zoonomia in 1794 a comprehensive theory of evolu- 

 tion, but did not support his views by many facts. 



