MODERN BIOLOGY 365 



dem Telle ein Ganges." This definition of fertilization is pure metaphysics, 

 here closely akin to Aristotle's. In opposition to Wolff's one-sided cpigenesis 

 theory, von Baer declares that in reality no new formation takes place in the 

 egg, but only transformation in the direction of increasing specialization. 

 True, this theory is also based on purely speculative grounds — that the 

 idea of the producing animal-form controls the development of the embryo — 

 but it at any rate leads to a result that has been accepted even in modern 

 times. Further, against Meckel's theory that during the embryonic stage 

 higher animals pass through the form of lower animals, von Baer makes one 

 particularly striking criticism; he maintains that no lower animals exist 

 that really resemble the embryonic stages of the higher animals, but, on 

 the other hand, the embryo of a higher animal and that of a lower animal 

 resemble one another more closely than do the fully developed animals. The 

 tissues of the embryo are less differentiated than those of the animal itself 

 and are therefore more like the tissues in lower animals; but a fish embryo is 

 from the very outset, and always remains, a fish, just as every vertebrate 

 animal's embryo is from the beginning a vertebrate animal. Since, then, 

 evolution involves a differentiation, the principle holds good that "the 

 more dissimilar two animal forms are, the further we have to go back in 

 evolutional history to find an agreement." The common primal form for all 

 animals is the simple cell-form, the form of the egg and of the first embryonic 

 stage. Starting from these considerations, von Baer emphatically rejects the 

 Bonnet-Lamarckian theory of a uniform chain of development in the animal 

 kingdom and instead associates himself with Cuvier's type theory, which he 

 further develops. He maintains that a series of animals can in respect of the 

 development of one organ be progressive, while another organ in the same 

 animal series is regressive, and that one animal within a lower type may 

 attain to a very high development in comparison with another form which 

 comes low down within a higher type — the bee and the fish are cited, with 

 the intelligence as the standard of comparison. Each organ, therefore, should 

 be judged not only according to its definitive form, but also with reference 

 to its evolutional history; the different animal types often possess organs 

 having the same function, but an entirely different origin. He predicts that a 

 comparative investigation of the different organs in the animal kingdom on 

 this basis will prove of great importance for the future, and this prediction 

 has of course been fulfilled. 



His natural philoso-phy 

 Side by side with these progressive ideas, and often curiously interwoven 

 with them, we find in von Baer a wealth of ideas of manifestly natural- 

 philosophical origin which must certainly seem highly grotesque to the 

 modern mind, but which nevertheless have undoubtedly had some influence 

 on the biological speculation that was to come. A number of these ideas he 



