150 THE PATH OF SCIENCE 



have been surpassed to this day, although others before him— 

 especially that unhappy Dutchman, Jan Swammerdam— had 

 done magnificent work on insect anatomy. Such men as these 

 show how wrong it is to adopt a cynical or contemptuous atti- 

 tude toward the biologists of the seventeenth and eighteenth 

 centuries. 



Trembley made a marvelously detailed study of the natu- 

 ral budding of his little fresh-water polyp. Hydra. He showed 

 how a small part of the body wall protrudes, develops new 

 parts, and becomes a new individual, which separates. His 

 work on this subject actually proved that there is a real 

 epigenesis or increase in complexity during development. 

 But he was influenced so much by the belief of his friend 

 and compatriot Charles Bonnet in preformation that he 

 never relinquished belief in it. Bonnet had shown that plant- 

 lice multiply without the intervention of a male parent. He 

 was struck by the high degree of development of the young at 

 birth and knew that in many insects each stage of develop- 

 ment is enclosed within the skin of the previous stage. He 

 generalized from these facts and imagined that each genera- 

 tion of organisms was folded up in a minute form within the 

 reproductive bodies of the previous generation. Develop- 

 ment, then, was only an unfolding, not a real increase in com- 

 plexity. Extending this idea still further, he imagined that 

 all subsequent generations were already folded up within the 

 first female of each species that existed on the earth. This 

 emhoitement of generation within generation was widely be- 

 lieved during the eighteenth century. Although Trembley's 

 observations were sufficient to disprove it, it was the writings 

 of the placid Caspar Wolff that at last made people reject 

 preformation and accept epigenesis. Working first at Halle 

 and later in St. Petersburg, Wolff showed that there is a gen- 

 uine increase in complexity in the development of both 

 plants and animals and not a mere unfolding of preformed 

 parts. His work was scarcely noticed until the beginning of 

 the nineteenth century, after his death. AV^olff paved the way 



