42 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



nature and number of which would be predetermined by the 

 molecular structure of the organism. 



h. The organism may have no intrinsic tendency to vary, 

 but variation may be brought about by the influence of con- 

 ditions external to it. And in this case, also, the variability 

 induced may be either indefinite or defined by intrinsic limi- 

 tation. 



c. The two former cases may be combined, and variation 

 may to some extent depend upon intrinsic, and to some ex- 

 tent upon extrinsic, conditions. 



At present it can hardly be said that such evidence as 

 would justify the positive adoption of any one of these views 

 exists. 



If all living beings have come into existence by the gradual 

 modification, through a long series of generations, of a pri- 

 mordial living matter, the phenomena of embryonic develop- 

 ment ought to be explicable as particular cases of the general 

 law of hereditary transmission. On this view, a tadpole is 

 first a fish, and then a tailed amphibian, provided with both 

 gills and lungs, before it becomes a frog, because the frog 

 was the last term in a series of modifications whereby some 

 ancient fish became a urodele amphibian; and the urodele 

 amphibian became an anurous amphibian. In fact, the de- 

 velopment of the embryo is a recapitulation of the ancestral 

 history of the species. 



If this be so, it follows that the development of any 

 organism should furnish the key to its ancestral history; and 

 the attempt to decipher the full pedigree of organisms from 

 so much of the family history as is recorded in their develop- 

 ment has given rise to a special branch of biological specula- 

 tion, termed phylogeny. 



In practice, however, the reconstruction of the pedigree of 

 a group from the developmental history of its existing mem- 

 bers is fraught with difficulties. It is highly probable that 

 the series of developmental stages of the individual organism 

 never presents more than an abbreviated and condensed sum- 

 mary of ancestral conditions ; while this summary is often 

 strangely modified by variation and adaptation to conditions ; 

 and it must be confessed that, in most cases, we can do little 

 better than guess what is genuine recapitulation of ancestral 

 forms, and what is the effect of comparatively late adapta- 

 tion. 



The only perfectly safe foundation for the doctrine of evolu- 

 tion lies in the historical, or rather archasological, evidence 



