xvi THE HISTORY OF ZOOLOGY 649 



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 discredited. 

 The work is, however, rather an encyclopedia than the exposition 

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

 ception 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 recognised : 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 Motu Cordis 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, 

 or 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 

 inaccurate. 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- 

 hoek and Swammerdam in Holland, Robert Hook and Nehe- 

 miah Grew in England. Malpighi made numerous histological 



