The Biogenetic Law 131 



trip in a deep tunnel. The eustachian tube permits the air 

 pressure on the inner surface of the eardrum and that on 

 the outside to become equaHzed. The whalebone whales 

 dive to such depths that if they had eustachian tubes the 

 high pressure would burst the eardrums. Curiously enough, 

 they have long ago given up this last vestige of the gill-cleft, 

 as we can judge from the fossil remains of the Eocene period. 

 Yet every new whale, in developing from an cg^ to a baby 

 whale, still goes through the motions of wrinkling up the 

 sides of its neck into make-believe gill slits, like other mam- 

 mals, and birds, and reptiles — and fish. 



The Biogenetic Law 



The facts of development among the vertebrates re- 

 ceived intensive study in the early half of the last century. 

 The foundations for comparative embryology were laid by 

 Carl Ernst von Baer in 1828. One of the first generalizations 

 that came from these comparative studies was the parallel- 

 ism in development among animals of similar type. In gen- 

 eral, it is true that the more alike two species are (in their 

 adult stages) the more alike are they also in the successive 

 stages of development. The more the adults differ, the earlier 

 in their development will the embryos diverge in appear- 

 ance and structure. This has been found to hold for all 

 groups of animals, not only the vertebrates. 



Under the influence of the enthusiasm for the evolution 

 doctrine there was formulated ^the " biogenetic law," which 

 says in effect that each individual in the course of develop- 

 ment passes through a series of stages corresponding to an- 

 cestral types. This principle was overworked to such an 

 extent that it finally became an impediment to research and 

 understanding rather than an aid. After all it is not true, 

 no matter what we may think of the origin of species, that 

 the human being, for example, in the course of its develop- 

 ment is at one point a fish or at another point an amphibian. 

 At most, we may say that at one stage in its development 



