IN 7 'RODUC TION 3 



indeed, has the ground been cleared for that close alliance of the 

 evolutionists and the cytologists which forms so striking a feature 

 of contemporary biology. We may best examine the steps by which 

 this alliance has been effected by an outline of the cell-theory, fol- 

 lowed by a brief statement of its historical connection with the evolu- 

 tion-theory. 



During the past thirty years, the theory of organic descent has 

 been shown, by an overwhelming mass of evidence, to be the only 

 tenable conception of the origin of diverse living forms, however we 

 may conceive the causes of the process. While the study of general 

 zoology and botany has systematically set forth the results, and in a 

 measure the method, of organic evolution, the study of microscopical 

 anatomy has shown us the nature of the material on which it has 

 operated, demonstrating that the obvious characters of plants and 

 animals are but varying expressions of a subtle interior organization 

 common to all. In its broader outlines the nature of this organiza- 

 tion is now accurately determined ; and the " cell-theory," by which 

 it is formulated, is, therefore, no longer of an inferential or hypo- 

 thetical character, but a generalized statement of observed fact which 

 may be outlined as follows : 



In all the higher forms of life, whether plants or animals, the 

 body may be resolved into a vast host of minute structural units 

 known as cells, out of which, directly or indirectly, every part is 

 built (Fig. i). The substance of the skin, of the brain, of the blood, 

 of the bones or muscles or any other tissue, is not homogeneous, as it 

 appears to the unaided eye. The microscope shows it to be an aggre- 

 gate composed of innumerable minute bodies, as if it were a colony 

 or congeries of organisms more elementary than itself. These elemen- 

 tary bodies, the cells, are essentially minute masses of living matter 

 or protoplasm, a substance characterized by Huxley many years ago 

 as the "physical basis" of life" and now universally recognized as the 

 immediate substratum of all vital action. Endlessly diversified in the 

 details of their form and structure, cells nevertheless possess a charac- 

 teristic type 6f organization common to them all ; hence, in a certain 

 sense, they may be regarded as elementary organic units out of 

 which the body is compounded. In the lowest forms of life the 

 entire body consists of but a single cell (Fig. 2). In the higher multi- 

 cellular forms the body consists of a multitude of such cells asso- 

 ciated in one organic whole. Structurally, therefore, the multicellular 

 body is in a certain sense comparable with a colony or aggregation of 

 the lower one-celled forms. 1 From the physiological point of view a 

 like comparison may be drawn. In the one-celled forms all of the 



1 This comparison must be taken with some reservation, as will appear beyond. 



