14 THE EVOLUTION THEORY 



Mammals, more than two hundred Birds, and many other American 

 animals. 



Again, in a quite different way, the naturalist's field of vision 

 was widened, namely, by the invention of the simple microscope, with 

 which Leeuwenhoek first discovered the new world of Infusorians, and 

 Swammerdam made his notable observations on the structure and 

 development of the very varied minute animal inhabitants of fresh 

 water. In the same century, the seventeenth, anatomists like Tulpius, 

 Malpighi, and many others extended the knowledge of the internal 

 structure of the higher animals and of Man, and a foundation was laid 

 for a deeper insight into the nature of vital functions by the discovery 

 of the circulation of the blood in Man and the higher animals. In 

 the following century, the eighteenth, this path of active research was 

 eagerly followed, and we need only mention such names as Rdaumur, 

 Rosel von Rosenhof, De Geer, Bonnet, J. Chr. Schaf er, and Ledermuller, 

 to be immediately reminded of the wealth of facts about the structure, 

 life, and especially the development of our indigenous animals, which 

 we owe to the labours of these men. 



All these advances, great and many-sided as they were, did not 

 at once lead to a renewal of the attempt of Empedocles to explain the 

 origin of the organic world. This was as yet not even recognized as 

 a problem requiring investigation, for men were content to take the 

 world of life simply as a fact. The idea of getting beyond the naive, 

 poetic standpoint of the Mosaic story of Creation was as yet remote 

 from the minds of naturalists, partly because they were wholly 

 fascinated by the observation of masses of details, but chiefly because, 

 first by the English physician, John Ray (died 1678), then by the 

 great Swede, Carl Linn^, the conception of organic ' species ' had been 

 formulated and sharply defined. It is true enough that before the 

 works of these two men ' species ' had been spoken of, but without 

 being connected with any definite idea ; the word was used rather in 

 the same vague sense as the word 'genus,' to designate one of the 

 smaller groups of organic forms, but without implying any clear 

 idea of its scope or of its limitations. Now, however, for the first 

 time, the term 'species' came to be used strictly to mean the 

 smallest homogeneous group of individual forms of life upon the earth. 

 John Ray held that the surest indication of a ' species ' was that its 

 members had been produce.d from the same seed; that is, 'forms 

 which are of difierent species maintain this specific nature constantly, 

 and one species does not arise from the seed of another.' Here we 

 have the germ of the doctrine of the absolute nature and the 



