History of the Theory of Heredity. 27 



directly opposed to his liypotliesis, for we can easily 

 conceive that after an indirect method of development 

 has been established it might be perpetuated by Buffun's 

 organic molecules, provided these are given off by the 

 parent organism at all stages of its life, and not simply 

 after it has reached its final form. 



There is, however, another class of phenomena of even 

 greater importance — the phenomena of variation. 



Buffon's hypothesis accounts for the resemblance be- 

 tween the child and the parents, but we now know that 

 the child is not exactly like its parents or even midway 

 between them, that animals and plants are born with 

 a tendency to lary, that this variation may affect any 

 part of the body, and that by the selection of these con- 

 genital variations the most profound changes of heredi- 

 tary strueture may be produced. 



The fact of congenital variation is as profound, as uni- 

 versal, and as characteristic of living things as the fact 

 of heredity, and the constant appearance of new varia- 

 tions is as fatal to pulfon's hypothesis of evolution as it 

 is to that of Bonnet. 



With the growth of the modern science of morphology 

 these hypotheses have been abandoned and the hypothe- 

 sis of epigenesis almost universally accepted in their 

 place. 



This hypothesis, first brought into notice by the re- 

 searches of Harvey and Wolff on the development of the 

 chick, has gradually assumed a more definite shape with 

 the progress of embryology, and has been especially 

 modified by the growth of the cell theory. 



In its modern form it may, for convenience of discus- 

 sion, be divided into two parts^a statement of the ob- 

 served facts, and an explanation of the origin of the 

 phenomena. 



