68 EMBRYOLOGY IN ANTIQUITY [pt. ii 



case. His words on this subject cannot be condensed. "But neither 

 can the (formative) agent be external and yet it must needs be one 

 or other of the two. We must try then to solve this difficulty, for 

 perhaps some one of the statements made (already) cannot be made 

 without qualification, e.g. the statement that the parts cannot be 

 made by what is external to the semen. For if in a certain sense they 

 cannot, yet in another sense they can." (Thus Aristotle does some 

 justice to the environment.) "It is possible, then, that A should 

 miove B and B should move C, that, in fact, the case should be the 

 same as with the automatic machines shown as curiosities. For the 

 parts of such machines while at rest have a sort of potentiality of 

 motion in them, and when any external force puts the first of them 

 into motion, immediately the next is moved in actuality. As, then, 

 in these automatic machines the external force moves the parts in 

 a certain sense (not by touching any part at the moment but by 

 having touched one previously) , in like manner also that from which 

 the semen comes or in other words that which made the semen, sets 

 up the movement in the embryo and makes the parts of it by having 

 touched first something though not continuing to touch it. In a way 

 it is the innate motion that does this, as the act of building builds 

 a house. Plainly, then, while there is something which makes the 

 parts, this does not exist as a definite object, nor does it exist in the 

 semen at the first as a complete part." This notion of the setting 

 in motion of a wound-up clock is substantially modern and underlies 

 the physico-chemical analysis of the developing embryo. It is really 

 striking to find Aristotle using the machine analogy in order to explain 

 himself, for he, of all biologists, emphasised the final cause in natural 

 operations. However, he soon returns to a more vitalistic attitude 

 in the succeeding section. "But how is each part formed?" he says. 

 "We must answer this by starting in the first instance from the 

 principle that, in all products of Nature or art, a thing is made by 

 something actually existing out of that which is potentially the same 

 as the finished product. Now the semen is of such a nature and has 

 in it such a principle of motion, that when the motion is ceasing 

 each of the parts comes into being and that as a part having life or 

 soul. . . .And just as we should not say that an axe or other instrument 

 or organ was made by the fire alone, so neither shall we say that foot 

 or hand were made by fire alone . . . .While, then, we may allow that 

 hardness and softness, stickiness and brittleness, and whatever other 



