500 THE GENERAL PRINCIPLES OF ZOOLOGY 



are so closely united that it is not possible, either by observation 

 or imagination, to determine where one ends or begins." 



Thus the seventeenth century paved the way for the great 

 advance made in natural science in the succeeding hundred 

 years ; it was marked by several important discoveries, and by 

 the great improvement of the microscope ; but after all the 

 greatest service which it rendered to science was the inaugura- 

 tion of the spirit of independent investigation and research into 

 the facts of nature, accompanied by an increasing disinclination 

 to accept mere assertions as facts. On the other hand, we must 

 remember that on the philosophical side, animals were held to 

 be the result of special creation and immutable ; the boldest 

 thinkers could venture merely to point out that variations did 

 occur, to suggest the mutual relationships of animals, and hesi- 

 tatingly to offer the theory that there might be laws of nature 

 sufficient to account for all natural phenomena without intro- 

 ducing any supernatural agency. 



The eighteenth century is marked by great advances in 

 knowledge along morphological lines. In nearly all the coun- 

 tries of Europe there were naturalists making dissections of 

 animals and drawing them ; their work was carefully done, and 

 their results of the greatest value, but we have not space to 

 review them here, and a mere enumeration of the names of 

 these scientists would be of little value. In the field of embry- 

 ology, however, we should note the German, Wolff, who in 

 1759 gave the most accurate description of the development 

 of the chick that had appeared up to that time ; he showed that 

 the growth of the embryo was due to an upbuilding of the vari- 

 ous organs, an epigenesis, and that there was no preformation, 

 as was believed by the majority of the scientists of that day. 

 Unfortunately his work and conclusions did not receive the 

 recognition they deserved until much later, for he was violently 

 opposed by another German, von Haller, an eminent physiolo- 

 gist, who firmly upheld the older theory of development. Much 

 was learned about the geographical distribution of animals dur- 

 ing this century, through such extended voyages as those of 

 Cook and Bougainville. In this way, too, Europeans became 

 familiar with many new animals, and valuable collections were 

 started. 





