938 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Modern microscopes have revealed no miniature of the adult in the egg, 

 nor has modern physiology found necessary an assumption of extra-physical 

 forces within living matter. With the increase of knowledge the old and 

 crude preformation of Haller and Bonnet and the speculative epigenesis of 

 Wolff and Harvey have given place to the new preformation and epigenesis 

 of the present time, and all modern theories of heredity may be classed in 

 the one or the other category or as intermediate between them. The mod- 

 ern advocates of preformation explain hereditary resemblance by the supposed 

 similarity of all germ-plasm in any one line of descent. The modern advocates 

 of epigenesis, while allowing the necessity of a material basis of germ-plasm, 

 ascribe hereditary resemblance to similarity of environment during develop- 

 ment. 



Variation. It is a commonplace in observation that, however close hereditary 

 resemblance may be, it is never absolute ; the child is never the exact image 

 of the parent either physically or mentally. Variations from the parental type 

 may be either acquired by the offspring subsequent to fertilization or to birth, 

 and hence are to be traced to the action of the environment; or they may be 

 congenital, that is, inherent in the germ-plasm. Although it is not always 

 easy in the case of any one variation to determine to which class it belongs, 

 yet the fact remains that the two classes exist; and a complete theory of 

 heredity must recognize and explain congenital variation as fully as congenital 

 resemblance. It is unnecessary to say that the origin of congenital variation 

 is one of the much discussed and still unsettled questions. At least two causes 

 of congenital variations are commonly recognized, although opinions differ as 

 to the relative importance of the r6le played by each. These causes are differ- 

 ences in the nutrition of the germ-plasm, and sexual reproduction. As to the 

 former, it is evident that the germ-plasm in no two individuals, even father 

 and son, has exactly identical nutritional opportunities. Since the life of one 

 individual is not the exact counterpart of the life of another, the germ-plasm 

 of one individual has a different nutrition from that of another. It would 

 hence be strange, even although we regard the germ-plasm as relatively stable, 

 if with succeeding generations there did not appear variations that are sufficient 

 to give rise to unlikeness in relatives. Differences in the nutrition of the germ- 

 plasm in different individuals are, therefore, a true cause of variations. As 

 regards sexual reproduction, it must be remembered that a new individual is 

 the product of two individuals, that the two individuals have descended along 

 different genealogical lines, and hence that the two conjugating masses of germ- 

 plasm are different in nature. It is only to be expected, therefore, that the 

 resulting individual shall be different from the two contributing parents. Thus 

 sexual reproduction is a true cause of variations. 



Having outlined the main facts and principles of heredity, let us now review 

 a few of the specific theories that have been of value in clearing the clouded 

 atmosphere. 



Darwin's Theory of Pangenem. Darwin's " Provisional Hypothesis of 

 Pangenesis" was published in 1868 as chapter xxvii. of his work on The Vari- 



