378 PROGRESS OF SCIENCE IN THE CENTURY. 



the facts carefully it is at once evident that the 

 larval frog (or tadpole in the wide sense) is never 

 a little fish, though it has undoubtedly a fish-like 

 heart, a fish-like circulation, and fish-like gills. It 

 is, none the less, from the very outset an amphibian, 

 and even more than that a frog ; whether we consider 

 its scaleless skin with multicellular skin-glands, or 

 its muscular tongue, or its rayless dorsal fin, or its 

 posterior nares, or a dozen other features, it is an 

 amphibian from beginning to end. The parallelism 

 is rather between the development and the phylogeny 

 of organs, than between the life-history and the evo- 

 lution of organisms. And even in regard to organs, 

 the recapitulation-doctrine in its cruder forms breaks 

 down, for in RabPs recent monograph on the lenses 

 of vertebrates, it is clearly shown that although in 

 the development of the higher lenses (of mammals, 

 for instance) there is some recapitulation of the evo- 

 lutionary stages, yet the earliest rudiment of the lens 

 (of a cat or of a bat) is specifically peculiar in every 

 case. 



Probably as the result of rapid development, the 

 generalisations of embryology such as the germ- 

 layer-theory, the gastrcea-theory, the recapitulation 

 doctrine, are no longer tenable without many 

 saving-clauses. But, since each, undoubtedly, ex- 

 presses some truth, our endeavour should be not that 

 of destructive criticism, but rather that of adapting 

 them to the new data. 



Physiological Embryology. What Pander and 

 Lotze suggested, that there should be an enquiry 

 into the immediate conditions which are operative in 

 development, was recognised by His in the famous 

 work Unsere Korperform und das Problem ihrer 

 Entstehung (1875), and by Rauber in his Formbil- 



