THE LAW OF RECAPITULATION. 43 



in the higher forms which have purely embryonic types of development. From 

 what has been said, it will be recognized that the likeness of the embryo to the. 

 adult lower form is a general morphological resemblance only, not an exact one, 

 and that therefore it is extremely difficult to infer from the embryonic organiza- 

 tion what the ancestral type was. Hitherto all phylogenetic inferences drawn 

 by embryologists have been largely speculative in character, and, it may be 

 added, have been more remarkable for their number and variety than for their 

 value. 



The resemblance between embryos and lower adult forms has been known 

 for a century past. It was first adequately asserted in 181 1 by J. F. Meckel, and 

 since then has been constantly discussed. More, perhaps, was done to empha- 

 size it by Louis Agassiz than by any one else. Von Baer, the creator of modern 

 scientific embryology, called attention in 1828 to the limitations which must 

 necessarily be put upon Meckel's generalization. It is to be regretted that von 

 Baer's wise thought on this subject has not been more appreciated. He put 

 forth four generalizations : First, that which is common to a large group of ani- 

 mals develops in the embryo earlier than that which is special; second, from 

 the most generalized stage structures less generalized are developed, and so on 

 until finally the most special appears; third, the embryo of a given animal form, 

 instead of passing through the other given forms, separates itself from them 

 more and more; fourth, therefore, essentially the embryo of the higher forms is 

 never like a lower form, but only like its embryo. The first to point out the 

 possible phylogenetic significance of these facts with perfect clearness was Fritz 

 Miiller, in a little book entitled "Fur Darwin," published in 1864. Ernst 

 Haeckel took up this interpretation and secured wider attention for it. He 

 termed the law of recapitulation the " biogenetic law."* 



The student will encounter in his practical study many illustrations of the 

 resemblances which we have been discussing, so that it is unnecessary here to do 

 more than mention a few for the purpose of illustration. In the embryos of 

 birds and mammals the pharynx forms a series of lateral pouches which we know 

 as the gill pouches, and which develop in the same way as, resemble strikingly, 

 and are homologous with, the gill pouches of fishes, which in the fishes give rise 

 to the so-called gill clefts. The heart of a young mammalian embryo is a simple 

 tube with only a single continuous cavity resembling the heart of the lower 

 fishes. The embryonic kidney or Wolffian body of man resembles, and is 

 homologous with, the kidney of the frog, but it disappears almost completely 

 before adult life. These few examples may suffice. 



* " Biogenetisches Grundgesetz." 



