2 IXTRODUCTION 



obvious characters of plants and animals — their forms, colours, 

 habits, distribution, their anatom\' and embryonic development — 

 and with the systems of classification based upon such characters ; 

 and long afterward it was, in the main, the study of like characters 

 with reference to their historical origin that led Darwin to his splen- 

 did triumphs. The study of microscopical anatomy, on which the 

 cell-theory was based, lay in a different field. It was begun and long 

 carried forward with no thought of its bearing on the origin of living 

 forms ; and even at the present day the fundamental problems of 

 organization, with which the cell-theory deals, are far less accessible 

 to historical inquiry than those suggested by the more obvious 

 external characters of ])lants and animals. Only within a few years, 

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

 students of organic evolution and students of the cell, which forms so 

 striking a feature of latter-day biology and is exerting so great an influ- 

 ence on the direction of research. It has, therefore, only recently 

 become possible adequately to formulate the great problems of devel- 

 opment and heredity in the terms of cellular biology — indeed, we can 

 as vet do little more than so formulate them. Yet the fact that these 

 two great lines of research, both concerned with the deeper problems 

 of life, yet having their beginnings so far apart, have at length 

 converged to a meeting-point, is one of the more striking evidences of 

 progress that modern biology has to show ; and it sufficiently justifies 

 an attempt to treat the cell from the standpoint of the general student 

 of development. 



Let us at the outset briefly outline the cell-theory as thus regarded, 

 and indicate the manner of its historical connection with the general 

 problems of evolution.^ 



^ Schleiden and Schwann are universally and justly recognized as the founders of the cell- 

 theory; hut like every other great generalization the theory was based on a long series of 

 earlier investigations l)eginniiig with the memorable microscopical researches of l.eeuwen- 

 hoek, Mali)ighi, lIo(>ke, and (jrew in the second lialf of the seventeenth century. 



Wolff, in the Theoria Cenerationis (1759), clearly recognized the "spheres" and "vesi- 

 cles" composing the embryonic parts both of animals and of plants, though without grasping 

 iheir real nature or mode of origin, and his conclusions were developed by the botanist 

 Mirbel at the beginning of the i^resent century. Nearly at the same time (1805) Oken fore- 

 shadowed the cell-theory in the form that it assumed with Schleiden and Schwann; but his 

 conception of " Urschleim " and " Hlaschen " can hardly be regarded as more than a lucky 

 guess. A still closer approximation to the truth is fuuntl in the works of 'ruri)in (1826), 

 Meyen (1830), Raspail (1831), and Dutrochet (1837); '^"^^ these, like others of the same 

 period, only paved the way for the real founders of the cell-theory. Among other immedi- 

 ate predecessors f)r contemporaries of Schleiden and Schwann should be especially mentioned 

 Robert Brown, Dujardin, Johannes Miiller, I'urkinje, Hugo von Mohl, Valentin, Unger, 

 Nageli, and Henle. The significance of Schleiden's, and especially of Schwann's, work lies 

 in the thorough and comprehensive way in which the problem was studied, the philosophic 

 breadth with which the conclusions were developed, and the far-reaching influence which 

 they exercised upon subsequent research. In this respect it is hardly too much to com- 

 pare the Mikroikopische L'ntersiichnngcn with the Origin of Species. 



