296 THEORIES OF INHERITANCE AND DEVELOPMENT 



approaching it we may well make a frank confession of ignorance ; 

 for in spite of all that the microscope has revealed, we have not 

 yet penetrated the mystery, and inheritance and development still 

 remain in their fundamental aspects as great a riddle as they were 

 to the Greeks. What we have gained is a tolerably precise acquaint- 

 ance with the external aspects of development. The gross errors of 

 the early preformationists have been dispelled. ^ We know that the 

 germ-cell contains no predelineated embryo ; that development is 

 manifested, on the one hand, by a continued process of cell-division, 

 on the other hand, by a process of differentiation, through which 

 the cells gradually assume diverse forms and functions, and so 

 accomplish a physiological division of labour. But we have not yet 

 fathomed the inmost structure of the germ-cell, and the means by 

 which the latent adult characters that it involves are made actual 

 as development proceeds. And it should be clearly understood that 

 when we attempt to approach these deeper problems we are com- 

 pelled to advance beyond the solid ground of fact into a region of 

 more or less doubtful and shifting hypothesis, where the point of 

 view continually changes as we proceed. It would, however, be an 

 error to conclude that modern hypotheses of inheritance and develop- 

 ment are baseless speculations that attempt a merely formal solution 

 of the problem, like those of the seventeenth and eighteenth cen- 

 turies. They are a product of the inductive method, a direct out- 

 come of accurately determined fact, and they lend to the study of 

 embryology a point and precision that it would largely lack if limited 

 to a strictly objective description of phenomena. 



All discussions of development are now revolving about two cen- 

 tral hypotheses, a preliminary examination of which will serve as 

 an introduction to the general subject. These are, first, the theory 

 of Germinal Localisation'^ of Wilhelm His ('74), and second, the 

 Idioplasm Hypothesis of Nageli ('84). The relation between these 

 two conceptions, close as it is, is not at first sight very apparent ; and 

 for the purpose of a preliminary sketch they may best be considered 

 separately. 



A. The Theory of Germinal Localization 



Although the naive early theory of preformation and evolution 

 was long since abandoned, yet we find an after-image of it in the 

 theory of germinal localization which in one form or another has 



1 Cf. Introduction, p. 6. 



2 I venture to suggest this term as an English equivalent for the awkward expression 

 " Organl)ildende Keimbezirke " of His. 



