464 ANIMAL BIOLOGY 



tion to the obviously inadequate explanations which chemistry 

 and physics could offer for the phenomena of irritability of living 

 matter then prominently engaging the attention of biologists. 

 The vitalists of that period abandoned almost completely all 

 attempts to explain life processes on a physico-chemical basis, and 

 assumed that an all-controlling, unknown, mystical, hyper- 

 mechanical force was responsible for all living processes. It is 

 apparent that such an assumption in such a form is a negation of 

 the scientific method, and at once removes the problem from the 

 realm of scientific investigation. 



Of course, no biologist at the present time subscribes to vitalism 

 in this form ; some uphold vitalism — if it must still be called vital- 

 ism — in its considerably limited modern form ; while all will un- 

 doubtedly admit that we are at the present time utterly unable 

 to give an adequate explanation of the fundamental life processes 

 in terms of physics and chemistry. Whether we shall ever be 

 able to do so is unprofitable to speculate about, though certainly 

 the twentieth century finds few scientists who really expect a 

 scientific explanation of life ever to be attained or who expect 

 that protoplasm will ever be synthesized. However that may be, 

 this much is positive: during the past fifty years some biologists 

 have now and then thought they were on the verge of artificially 

 creating life in the test tube, only to leave the problem, like the 

 alchemists of old, with more respect for the complexities of proto- 

 plasmic organization and the enormous gap which separates even 

 the simplest forms of life from the inorganic world. 



4. Histology 



Studies on the physiology of plants and animals naturally 

 involved the progressive analysis of the physical basis of the phe- 

 nomena under consideration, but the Aristotelian classification of 

 the materials of the body as unorganized substance, homogeneous 

 parts or tissues, and heterogeneous parts or organs, practically 

 represented the level of analysis until the beginning of the eight- 

 eenth century. In fact it was not until the revival of interest in 

 embryology early in the last century that the cell became a particu- 

 lar object of study, and attention began gradually to shift from 

 more or less superficial details to cell organization. This culminated 

 in the classic investigations of two German biologists, the botanist 

 Schleiden (1804-1881) and the zoologist Schwann (1810-1882), 



