1298 THE STORY OF THE UNIVERSE 



Anchitherioid ancestor, through the form of Hip- 

 parion, appears to admit of no doubt. And all the 

 facts of geology and palaeontology not only tend to 

 show that the knowledge of ancestral development 

 is likely long to remain fragmentary, but lead us to 

 doubt whether even such fragments as may be vouch- 

 safed to us by the extension of geological inquiry will 

 ever be sufficiently old, in relation to the whole 

 duration of life on the earth, to give us positive evi- 

 dence of the nature of the earliest forms of animals. 



In the case of an existing animal, it is possible to 

 determine its adult structure and its development, 

 and therefore to assign its place relatively to other 

 animals, the structure and development of which are 

 also known ; and, in the case of an extinct animal, it 

 is possible to ascertain certain facts of its structure, 

 and sometimes certain facts of its development, which 

 will justify a more or less positive assignment of 

 its place relatively to existing animals. So far, Tax- 

 onomy is objective, capable of proof and disproof, 

 and it should leave speculation aside, until specula- 

 tion has converted itself into demonstration. 



In the present rapidly shifting condition of our 

 knowledge of the facts of animal structure and de- 

 velopment, however, it is no easy matter to group 

 these facts into general propositions which shall ex- 

 press neither more nor less than is contained in the 

 facts; and no one can be more conscious of the mani- 

 fold imperfections of the following attempt at such 

 a classification than the author of it. 



In certain of the lower animals, the substance of 

 the body is not differentiated into histogenetic ele- 



