LIMITING FACTORS IN THE HISTORY OF SCIENCE 



held during the eighteenth century would not perhaps have been so 

 crushing if the biologists of that time had been able to take a 

 mathematical argument more seriously. There was Harvey's very con- 

 vincing argument about the circulation of the blood, and Freind's^ 

 equally convincing (though unfortunately in tendency erroneous), 

 argument about the quantity of menstrual blood and the weight of the 

 newborn foetus. Verbally, it was still quite possible to support the 

 Hellenistic view that the embryo was formed from menstrual blood, 

 in the post-Harveian period, if it were admitted that this blood flowed 

 little by little through the umbilical vessels. This was the position of 

 John Freind in his treatise on menstruation, Emmenologia (1700-30). 

 Calculating the amount evacuated in nine months, he said: "The 

 quantity of Blood which the Mother may bestow upon the nourish- 

 ment of her Offspring will be lib. i^ § 2J, which will outweigh the 

 newborn Foetus with all its Integuments, if they should be put into 

 a Balance; and leave no room to doubt, its being able to bestow 

 very proper nourishment on the Embrio. For the mean weight of a 

 new-born Foetus is about /ib. 12, some-times it is found greater, 

 and very often less." 



If these could have been accepted, it was a pity that Hartsoeker's 

 argument about preformation could not. In 1722 Hartsoeker^ cal- 

 culated that lo^^'^'^o^ rabbits must have existed in the first rabbit, assu- 

 ming that the creation took place 6,000 years ago and that rabbits begin 

 to reproduce their kind at the age of six months. But to this Bonnet 

 merely answered that it was always possible, by adding zeros to units, 

 to crush the imagination under the weight of numbers, and he de- 

 scribed the performation theory as one of the most striking victories 

 of the understanding over the senses. It would have been better 

 described as one of the most striking victories of the imagination 

 over the understanding. 



The fact is that the biologists of the eighteenth century, carried 

 away by preformationist theory, took embryology on to a plane 

 where observation became superfluous. They would have found 

 acceptable the sentiment satirised by Boyle that "it is much more 

 high and philosophical to argue a priori than a posteriori,'' and were 

 eventually debarred from looking at developing embryos by their 

 conviction that structure and organisation would certainly be there, 

 whether they could see it or not. The preformationist controversy 

 was, in fact, a repetition in biology of the controversy between the 



^ Emmenologia, 1720. ^ Receuil de plusieurs pieces de physique, 1722. 



