630 ZOOLOGY SECT. 



During the same period the general knowledge of animals was 

 increasing, and a distinct epoch is marked by the learned, and, for 

 the time, exhaustive Historia animalium of Conrad Gesner, 

 published in 1551-58, and consisting of 4,500 folio pages, with 

 numerous illustrations, some of them of considerable merit, some 

 wonderfully inaccurate, some depicting various fabulous monsters, 

 such as Winged Dragons, many-headed Hydras, and crowned 

 Basilisks, the existence of which was not yet thoroughly dis- 

 credited. The work is, however, rather an encyclopaedia than the 

 exposition of a science : it contains no general ideas ; there is still 

 no conception of the subordination of groups, and no exact naming 

 either of animals as a whole or of .their various parts. Five 

 chief groups of animals are recognized : Viviparous Quadrupeds, 

 Oviparous Quadrupeds, Birds, Aquatic Animals, and Serpents. 

 Within these divisions the various animals are described without 

 any attempt at grouping. Among Aquatic Animals, for instance. 

 Fishes, Amphibia, Cetacea, Molluscs, Crustacea, Echinodermata, 

 and Sea-serpents are included. 



In the seventeenth century great strides were made both in 



knowledge of structure and function, in generalisation, and in 



methods of investigation. Especially famous and fruitful indeed 



one of the greatest scientific events of all time was the discovery 



of the circulation of the blood, made by William Harvey in 1616, 



and announced in 1628 in a small pamphlet Exercitatio anatomica 



de Motif, Corclis et Sanguinis. He demonstrated fully, partly by 



dissections, partly by experiments on living animals, the action 



of the heart as a pumping mechanism, the nature of its valves and 



of those of the veins, the presence of blood, not air, as was then 



supposed in the arteries, the cause of the pulse, and the whole 



course of the circulation so far as it could be known previous to 



the discovery of the microscopic capillaries. Of hardly less 



importance is Harvey's embryological work : he made extended 



observations on the development of the Chick and in his Exercita- 



tiones de Generatione Animalium (1657) declared that all living 



things arise from a primordium, or ovum, and propounded the 



doctrine of epigenesis according to which development is a process 



of gradual differentiation of the primordium, whereby " out of the 



inorganic arises the organic, out of the similar the dissimilar." 



The primordium itself he considered might " proceed from parents, 



< >r arise spontaneously, or out of putrefaction." 



Harvey worked with no optical aid beyond a simple lens, and it 

 is not surprising that his results are incomplete and often in- 

 accurate. His successors had the advantage of the compound 

 microscope, invented by Hans and Zacharias Janssen about 

 1590-1600, and sufficiently improved during the course of the 

 seventeenth century to be an important engine of research in the 

 hands of the earliest microscopists, Malpighi in Italy, Leeuwen- 



