THE HISTORY OF EVOLUTIONARY THOUGHT 



His studies in comparative embryology led him to broaden the principles 

 of von Baer to make the Biogenetic Law, in support oi which he published 

 extensively. He based extensive phylogenies upon eml^ryological evidence, 

 interpreted according to the Biogenetic Law. Bateson has said that this 

 "law" dominated all of the zoology of the last half of the nineteenth 

 century. Haeckel's bona fide scientific work was all done in his youth. In 

 later life, he became primarily a controversialist and popularizer, a fact 

 which is said to have earned him the contempt of Gegenbaur. 



August Weismann's first interest was heredity, the aspect of Darwinism 

 which Darwin himself had recognized as weakest. Weismann (Figure 28) 

 also seems not to have heard of Mendel and his work. He was hampered 

 by progressive blindness which became complete before his major works 

 were done. Over much of his career, his graduate students made obser- 

 vations and reported them to him in detail. His data were principally 

 some of the facts of cytology, mainly of mitosis. He reasoned that, since 

 the hereditary mechanism must be orderly, and since only the chromo- 

 somes were divided in an exact and orderly fashion in mitosis, the chro- 

 mosomes must be the phvsical basis of heredity. The facts of meiosis were 

 not yet known, but he predicted reduction divisions because otherwise 

 the chromosomes would be doubled in number from one generation to the 

 next, an unstable situation. Beyond these propositions, which may have 

 been proven since, his hypothesis of heredity was purely speculative, and 



Figure 28. August Weis- 

 mann. ( From Locy, "Bi- 

 ology and Its Makers," 

 3rcf Ed., Henry Holt & 

 Co., Inc., 1935. ) 



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