SPECULATIVE ZOOLOGY. 3 6 5 



or of a class or order are exhibited in their simplicity, and uncom- 

 plicated by the presence of the characteristics of any of the minor 

 divisions of the group : thus amphioxus is a generalized or diagram- 

 matic vertebrate, with structural features which are common to the 

 whole group, and none of the distinctive marks of any of the great 

 subdivisions of the group. In a phylogenetic tree such a form would 

 be represented by a line running almost directly upward from a point 

 where great branches diverge from a common stem ; and the fact that 

 these generalized forms are much more often found among fossil than 

 among recent animals is very suggestive. 



This short analysis is sufficient to show that the essential similarity 

 between a system of classification based on homology and a phylo- 

 genetic tree is so great that all objections to the one method of gen- 

 eralizing from the facts must apply to the other. If phylogenetic 

 speculations retard science, speculations upon homology must do the 

 same thing, and the only way to avoid danger will be to stick to 

 facts, and, stripping our science of all that renders it worthy of think- 

 ing men, to become mere observing machines. 



As it is plain that the strictest construction of the proper scope 

 and limits of scientific thought does not make any such demands as 

 this, we may feel at liberty to speculate upon phylogeny with such 

 a basis as is furnished by the comparative study of the systematic 

 relationship of living animals, but we are not able to go very far in 

 this direction, for nearly all living animals fall into a few great well- 

 defined groups, and generalized types are the exception. Our conclu- 

 sions must, therefore, be very vague and general, and we turn to the 

 paleontological record for more exact data ; but here, again, we soon 

 meet with limitations which prevent any very exact and definite gener- 

 alizations. One of these limitations, the imperfection of the record, 

 we have already examined, and another is due to the fact that most 

 of the main stems of the phylogenetic tree, and the minor branches of 

 many of them, were established before the time of the oldest fossil- 

 iferous rocks, and we can not hope to find fossil remains of the unspe- 

 cialized ancestors from which they were derived. This renders it 

 hopeless for us to attempt actual proof of the more deep-seated phylo- 

 genetic relationships ; and another consideration, which we will now 

 examine, renders the discovery of the exact relationship of smaller 

 groups of genera and species almost equally hopeless, even when the 

 most ample series of fossils is discovered. 



All that we know of variation indicates that it is not induced in 

 the adult, but that it is congenital, and the effect of some force which 

 has acted upon or through the parents. Hence it happens that, when 

 a new variety is produced, it is not usually by the addition of some- 

 thing new to the characteristics of a mature animal, but by a diver- 

 gence which shows itself before maturity is reached. When we com- 

 pare two closely related species or varieties of birds, we do not find 



