370 RECORDS. 



tion that the embryo, though invisible, really exists preformed 

 in the egg before development begins. The preformed germ 

 was not conceived to be an exact miniature model of the adult. 

 On the contrary. Bonnet thought of the germ of the fowl, for 

 example, as differing widely in form and proportions from an 

 actual bird, still the original preformation was assumed to be 

 composed of parts that correspond, each for each, to the parts 

 of the chick. Development, accordingly, was conceived to be 

 only the unfolding and transformation of a preexisting structure, 

 not the successive formation of new parts — a process of "evo- 

 lution," not of epigenesis. In this particular form the doctrine 

 of preformation was conclusively overthrown by Wolff; but the 

 principle underlying it has repeatedly and persistently reap- 

 peared in later speculations on development, and still contests 

 the field of discussion with its early antagonist. 



Hand in hand with this controversy has gone one of still more 

 general scope between the two opposing conceptions that I have 

 referred to as mechanism and vitalism. Is development at 

 bottom a mechanical process ? Is the egg a kind of complex 

 machine, wound up like a piece of clockwork, and does develop- 

 ment go forward like the action of an automaton, an inevitable 

 consequence of its mode of construction ? Or, on the other 

 hand, does development involve the operation of specific vital 

 entelechies or powers that are without analogue in the automaton 

 and are not inherent in any primary material configuration of the 

 egg ? This question, I hardly need say, is included in the larger 

 one, whether the vital processes as a whole are or are not 

 capable of mechanical explanation. As a problem of embry- 

 ology it is very closely connected with that of preformation or 

 epigenesis, and in point of fact the two have always been closely 

 associated. Evidently, by its very form of statement, any theory 

 of preformation or prelocalization in the germ assumes at least 

 a mechanical basis for development, /. e., a primary material 

 configuration upon which the form of development in some 

 measure depends. With theories of epigenesis the case is not 

 so clear ; for such theories may or may not be mechanical. 

 Without further preamble I now ask your attention to certain 



