ANIMALS AND THEIR ENVIRONMENTS. 299 



conditions of their life. Nor need we look far afield to 

 discover the reasons which induced naturalists to credit the 

 living part of Nature with a fixity which nowhere held sway 

 in the inorganic world. The tendency of biological opinion 

 in the past was to regard the forms of animal and plant life 

 as fixed quantities, which varied now and then, no doubt, 

 but which on the whole preserved, as far as observation 

 could detect, a perfect and stable uniformity of form and 

 function. With the extreme prevalence of the idea of the 

 fixity of animal and plant species, the doctrine of " special 

 creation " had unquestionably much to do. A glance at a 

 natural history text-book of some twenty years back or so, 

 will serve to show clearly and unmistakably that the former 

 idea of a " species " of animals or plants was based on the 

 continued and unvarying likeness of a number of living 

 beings to each other. Buffon's definition of a " species," for 

 example, shows that he regarded it as " a constant sucession 

 of individuals similar to and capable of reproducing each 

 other." And another authority, Miiller, defines species to 

 be "a living form, represented by individual beings, which 

 re-appears in the product of generation with certain invari- 

 able characters, and is constantly reproduced by the genera- 

 tive act of similar individuals." Thus the various species of 

 animals and plants were regarded as essentially immutable 

 in their nature, and as continuing permanently in the like- 

 ness which they had inherited from the creative fiat in the 

 beginning of this world's order. 



But meanwhile, ideas of a widely different nature regard- 

 ing the nature of living beings had been slowly asserting 

 themselves, and had their part outcome in the work of 

 Lamarck, who clearly recognised the effects of use and 

 disuse and of habit on the frames of animals, in producing 

 modifications of their form and structure. Similar or analo- 

 gous thoughts were beginning to influence the sister science 

 of geology. The writings of geologists who, like Hutton, 

 Playfair, and Lyell, advocated the doctrine of Uniformity 

 in opposition to that of an ill-defined Catastrophism, had a 



