CHAPTER IX 



THEORIES OF INHERITANCE AND DEVELOPMENT 



" It is certain that the germ is not merely a body in which life is dormant or potential, 

 but that it is itself simply a detached portion of the substance of a preexisting living body." 



HUXLEY. 1 



" Inheritance must be looked at as merely a form of growth." DARWIN. 2 



" Ich mochte daher wolil den Versuch wagen, durch eine Darstellung des Beobachteten 

 Sie zu einer tiefern Einsicht in die Zeugungs- und Entwickelungsgeschichte der organischen 

 Korper zu fiihren und zu zeigen, wie dieselben weder vorgebildet sind, noch auch, wie man 

 sich gewohnlich denkt, aus ungeformter Masse in einem bestimmten Momente plotzlich 

 ausschiessen." VON BAER. 3 



EVERY discussion of inheritance and development must take as its 

 point of departure the fact that the germ is a single cell similar in its 

 essential nature to any one of the tissue-cells of which the body is 

 composed. That a cell can carry with it the sum total of the heritage 

 of the species, that it can in the course of a few days or weeks give 

 rise to a mollusk or a man, is the greatest marvel of biological science. 

 In attempting to analyze the problems that it involves, we must from 

 the outset hold fast to the fact, on which Huxley insisted, that the 

 wonderful formative energy of the germ is not impressed upon it 

 from* without, but is inherent in the egg as a heritage from the paren- 

 tal life of which it was originally a part. The development of the 

 embryo is nothing new. It involves no breach of continuity, and is 

 but a continuation of the vital processes going on in the parental 

 body. What gives development its marvellous character is the rapid- 

 ity with which it proceeds and the diversity of the results attained in 

 a span so brief. 



But when we have grasped this cardinal fact, we have but focussed 

 our instruments for a study of the real problem. How do the adult 

 characteristics lie latent in the germ-cell ; and how do they become 

 patent as development proceeds ? This is the final question that looms 

 in the background of every investigation of the cell. In 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 fun- 



1 Evolution, Science and Culttire, p. 291. 



2 Variation of Animals and Plants, II., p. 398, 



3 Entwick, der Thiere, II., 1837, p. 8. 



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