SEVENTEENTH AND EIGHTEENTH CENTURIES 151 



induced by the " perfecfum nutrimentum" with which the pollen' provides the 

 seeds; lengthy arguments are brought forward to show why the pollen is 

 and must be the most perfect form of nourishment that exists. Sexual re- 

 production, then, is nothing more than a renewed growth. 



The fundamental principles on which growth proceeds in the vegetable 

 kingdom Wolff discovers in detail in the animal kingdom; in the embryo 

 of the chicken, which is his only subject of investigation in animal embry- 

 ology, he finds reproduced the same phenomenon of growth; the inner 

 force derives nourishment from the yolk of the embryonic lamina, this ali- 

 mental fluid coagulating, as in the plant, into ampullar and ducts, the latter 

 here represented by heart and vesicular system. Here, too, the details of the 

 embryonic development, which is described much less fully than that of 

 the plant, are of no particular interest; here again Wolff gives full rein to 

 his speculative imagination at the expense of detailed observation. Such an 

 assertion as that no one has discovered anything with a powerful magni- 

 fying-glass that could not equally well have been observed with a lower 

 magnification is sufficient evidence of how his speculations are out of accord 

 with reality. And still worse is his habit of comparing the structure of plants 

 and animals in detail; his comparison of a plant's vessels with the arteries, 

 of its suckers with the veins, rivals in absurdity most of what had hitherto 

 been perpetrated in that sphere — which was by no means little. 



And yet it is just through his comparison of plant and animal develop- 

 ment that Wolff made his most important contribution to biological history. 

 He was the first to compare the elements of which the plant and the animal 

 are composed, and though the details of this comparison are for the most 

 part incorrect, it was at any rate he before anyone else who pointed out the 

 ampullar -like structure — in other words, the cell-tissue — that is common 

 to both. He thus carried science a considerable step further along the road 

 marked out by Malpighi and his immediate successors. 



His epigenesis doctrine 

 Wolff's second service to science is generally said to be his introduction 

 of the doctrine of epigenesis into biology in place of the preformation theory. 

 We have previously found that the epigenesis theory is actually older than 

 the preformation theory; even Aristotle was an epigenetic, and his doctrine 

 was promulgated without contradiction even by Harvey, whereas the first 

 champion of the preformation theory was Swammerdam. It was thus an 

 ancient theory that Wolff adopted, and from the very outset he made it his 

 own on purely theoretical ground; it was only natural, therefore, that his 

 observations should eventually accord with the preconceived ideas. But the 

 progress of science was facilitated by the fact that — whether with pre- 

 conceptions or not — he saw more correctly in his microscope than his con- 

 temporary preformationists; for their part, they considered an embryological 



