SPECULATIVE ZOOLOGY. 377 



reason is there for regarding one set of resemblances as of taxonomic 

 importance rather than the other ? The answer is plain. It is easy 

 to show that all the features in which a whale resembles fishes are such 

 as we should expect to find if the whale is a mammal, adapted to an 

 aquatic life ; but the features of resemblance to an ordinary mammal 

 do not admit of any such explanation, and they must therefore be held 

 to indicate the true relationship. 



If the crawfish originally passed through stages somewhat similar 

 to those of the growing lobster, we can see why they may have been 

 suppressed to adapt it to a life in fresh water ; but, if the life-history 

 of the crawfish is ancestral, we can find no reason, in the life of the 

 lobster, for the acquisition of larval stages which are like those of 

 more distantly related macroura, and, in rejecting one life-history and 

 accepting the other, we are simply carrying the accepted principles of 

 homological reasoning into wider fields, and applying them to a new 

 class of phenomena ; and a thorough acquaintance with the facts will 

 render our conclusions as thoroughly scientific in the one case as in 

 the other. 



Those who are unfamiliar with the status of modern morphology 

 are still accustomed to regard systematic zoology as a science of obser- 

 vation, but our review of the subject shows that the attempt to trace 

 out the natural system of classification of animals carries us far beyond 

 the bare facts, and that the observed phenomena, although practically 

 infinite in numbers, bear about the same relation to the generalizations 

 of the science that the facts of mathematics or of astronomy do to the 

 general laws of these sciences. 



The facts are so numerous and so difficult to observe, and our ac- 

 quaintance with the conditions of life is so slight, that our attempts 

 at general conclusions must frequently be tentative and provisional, 

 and in some cases future research may show that they are entirely 

 wrong ; but this is no valid objection to the use of such evidence as 

 we have. There is no more justice in the assumption that, because 

 they may possibly be wrong, phylogenetic speculations upon the basis 

 of paleontology, comparative anatomy, and embryology are adverse to 

 the best interests of science, than there would be in the assumption 

 that the attempt to trace the relationship of animals from the facts of 

 homology is unscientific, because Cuvier thought that he had discovered 

 homologies between the barnacles and mollusks, or because Agassiz 

 associated the vorticellas with the polyzoa. 



The end of phylogenetic speculation is perfectly legitimate, but we 

 must rid our minds of the belief that it can be reached by mere obser- 

 vation and description. The evidence is often so hard to read that 

 the accounts of the best observers are contradictory, and in many cases 

 it is so scanty and incomplete that it must be submitted to a severely 

 critical process of comparison, analysis, cross-examination, and elimina- 

 tion, before a general conclusion can be reached. The field is so vast, 



