l66 THE HISTORY OF BIOLOGY 



and illustrated what he saw; if at any time he starts theorizing, he generally 

 fails, but usually he appears conscious of his limitations and holds to the 

 realities which he knew so well how to master. 



Biology has Leeuwenhoek to thank for a long series of facts of funda- 

 mental importance. His studies on the circulation of the blood deserve first 

 consideration. He has explained and completed the knowledge of the capil- 

 lary system which Malpighi originated, while he clearly proved that the 

 veins and arteries, each separately, are continued on immediately through the 

 capillaries and thus through them merge directly into one another. More- 

 over, Leeuwenhoek for the first time clearly recognized the blood corpuscles 

 and described them, first in the frog and then in man and a number of animal 

 forms. Malpighi had thought that he could distinguish in the blood "fat 

 globules," but he did not investigate the matter further, so that to Leeu- 

 wenhoek is due the honour of having really solved the problem. The same 

 is the case with the spermatozoa, which, it is true, a Dutch student by the 

 name of Hamm was the first to observe, but which Leeuwenhoek at any rate 

 studied more closely in a number of animal forms. In this connexion he made 

 thorough investigation into the fertilization of various animals, especially 

 fishes and frogs. In the frog he noted the spermatozoon's association with 

 the egg and believes — like Aristotle, as a matter of fact — that it is from 

 the male that the actual life comes; the female only provides, through the 

 egg, nourishment and powers of development. This he tries to prove by pair- 

 ing different-coloured rabbits : if a white female is paired with a grey male, 

 all the young will be grey like the father. Had he continued the experiment 

 through several generations, he would certainly have obtained other results. 

 — He has further observed a number of histological details of various kinds: 

 the stripes of the striated muscles, the structure of dental bone, the construc- 

 tion of the optic lens in man and the higher animals. No less remarkable are 

 his discoveries in the lower animal world. He thus discovered the Infusoria 

 and the Rotatoria in water; he explained the reproduction of ants, found 

 their true eggs, and showed that what had hitherto been called ants' eggs 

 are really the pupas of the insect. He very definitely opposes the hitherto 

 prevalent view that minute creatures of all kinds arise through putrefaction 

 or fermentation in inanimate matter. Instead he declares that even the small- 

 est animals possess reproductive powers and propagate solely by means of 

 them. In proof of this he demonstrated particularly the evolution of fleas 

 and aphids. If, finally, we add to this that he demonstrated the difference 

 between the structure of the stem of monocotyledons and dicotyledons in the 

 vegetable kingdom, this — by no means complete — sketch will have given 

 some idea of a life of activity which, without making any important theo- 

 retical contributions, advanced the knowledge of nature in an unusually high 

 degree. 



