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 pre-existing living body." 



HUXLEY.l 



" Inheritance must be looked at as merely a form of growth." Darwin.^ 



" Ich mochte daher wohl 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.'^ 



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, w^e 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 parental 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 pro- 

 cesses going on in the parental body. What gives development its 

 marvellous character is the rapidity 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. Ho:<:' 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 



^ Evolution, Science and Culture, p. 291. 

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

 ^ Etttwick. der Thiere, II., 1837, p. 8. 

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