THE GROWTH OF BIOLOGICAL IDEAS 149 



actual fertilizing bodies. Reproduction could not be seri- 

 ously investigated until it was known for certain whether 

 organisms arise only from pre-existent organisms or whether, 

 on the contrary, they are sometimes spontaneously generated 

 from non-living matter. Harvey himself in 1651 announced 

 that every organism originates from an egg (though he never 

 saw the tgg of mammals); and ten years later Redi, physician 

 at the court of Florence, showed experimentally that larvae 

 appear in rotting meat only if flies lay eggs on it. That re- 

 markable man John Needham, an English Catholic priest 

 living on the continent, performed experiments nearly a cen- 

 tury afterward that caused him to be a firm believer in spon- 

 taneous generation. Toward the end of the eighteenth cen- 

 tury Spallanzani boiled various organic materials in airtight 

 containers and showed that life did not originate in them. 

 His experiments were so carefully done that they might have 

 settled the matter, but, as we shall see (page 166), the subject 

 was raised again much later. The Mammalian egg was first 

 seen in 1827 by the Esthonian K. E. von Baer, who also 

 made marvelously exact studies of the development of various 

 animals and may be regarded as the father of modern de- 

 scriptive embryology. 



It is not only from eggs, however, that animals arise. This 

 had been shown toward the middle of the eighteenth century 

 by a Genevese naturalist, Abraham Trembley, who was acting 

 as tutor in a family living near The Hague. Trembley ob- 

 served some remarkable polyps in water taken from a ditch 

 and studied them with such profundity that his work is quoted 

 in modern textbooks not as a historical curiosity but for its 

 sound information on an important subject. He was the 

 first to show that certain animals can be multiplied artifi- 

 cially by cutting them into pieces, and he made a careful 

 study of the processes of regeneration. His friend Lyonet, a 

 Frenchman living at The Hague, made equally exact studies 

 in a different field. His description of the anatomy of the 

 goat-moth caterpillar is an example of accuracy and careful 

 observation that is thought by many good judges never to 



