40 FATHERS OF BIOLOGY. 



secretion : thus the hand, or the face, or the whole 

 animal pre-exists in the sperm, though in an undifferen- 

 tiated state (dSio/no-rus) ; and what each of these is in 

 actuality (ci/epyetp), such is the sperm in potentiality 



In later times the two great rival theories put for- 

 ward to account for the development of the embryo 

 have been 



(a) The theory of Evolution, which makes the embryo 



pre-existent in the germ, and only rendered 

 visible by the unfolding and expansion of its 

 organs. 



(b) The theory of Epigenesis, which makes the em- 



bryo arise, by a series of successive differentia- 

 tions, from a simple homogeneous mass into 

 a complex heterogeneous organism. 

 The above quotation will show how closely Aristotle 

 held to the theory of Epigenesis ; and in another place 

 he says, " Not at once is the animal a man or a horse, 

 for the end is last attained; and the specific form is 

 the end of each development." 



Spontaneous generation is nowadays rejected by 

 science ; but Aristotle went so far as to believe that 

 insects, molluscs, and even eels, were spontaneously 

 generated. It is, however, noteworthy, in view of modern 

 investigations, that he looked upon putrefying matter as 

 the source of such development. 



