226 BIOLOGICAL LECTURES. 



vision. It is the vantage-ground of progress, the conquest of 

 laborious research, of which one might say, as Johannes 

 Miiller once said of his own work : " Es klebt Blut an der 

 Arbeit'" We have to deal, then, with a question of moment, 

 and one which presents, in addition to its inherent difficulties, 

 the obstacles raised by prejudgment. Let us try to clear the 

 ground a little, so as to get into closer touch with the question. 



One fact orients the whole field. It is the fact that we now 

 build upon two broad truths which found their negation in the 

 old theories of development, namely, heredity and generation. 

 It may sound a little paradoxical, but it is true, that the two 

 theories of last century not only contradicted each other, but 

 also denied the very truths they came to explain. Evolution 

 was the absolute negation of both heredity and generation, 

 while epigenesis upheld generation, but denied organic con- 

 tinuity, the essential foundation of heredity. Let us make no 

 mistake on this point, for it is fundamental and decisive as 

 regards standpoints. 



Both Bonnet and Haller boldly denied the possibility of gen- 

 eration. Why ? For the obvious reason that generation meant 

 epigenesis. There was no middle ground. If by any possibil- 

 ity anything of an organic nature could be referred to epigene- 

 sis, the miracle of creation would be reduced to the level of an 

 every-day occurrence. The backbone of the argument for 

 original preformation would go to pieces if a single vertebra 

 could arise epigenetically. Not so much as a supernumerary 

 digit, or a monstrous organ of any description, troublesome as 

 such things were to the preformationist, could be allowed to 

 pass to the credit of epigenesis. Allow that a single organ can 

 be formed anew, and the whole edifice of preformation would 

 be irretrievably undermined. Bonnet saw the bearings and the 

 perils of his theory, and he did all that ingenuity could do to 

 guard the central idea against hostile attacks. 



What that central idea was, and how the fate of the whole 

 theory hung upon it, Bonnet makes clear in one of his earlier 

 writings. Referring to the principles advanced in relation to the 

 development of the mule, Bonnet makes the following remarks, 

 " prophetic of the event" already fulfilled on his own head: 



