1897- , MORPHOLOGY IN ZOOLOGICAL SCIENCE. 31 



the type of a family by itself. Amongst the vertebrates we find 

 numerous examples wherever we choose to look. The Teleostei are 

 polyphyletic, and so are the ganoids ; so marked does the parallelism 

 in evolution seem to have been, that our ideas as to the mutual 

 relationships of the two groups are in hopeless confusion. Snake-like 

 reptiles have been derived from many families of lizards. Huxley 

 arbitrarily selected the characters of the absence of all trace of a 

 pectoral girdle, and of an allantoic bladder as a criterion to determine 

 the true snakes ; but it is obvious that this is a mere makeshift. 

 Among birds, parallelism is seen everywhere, and instances of it can 

 be found in the works of Gadow and Furbringer. 



Turning finally to the Mammalia, we find Sir William Flower 

 writing thus of the Ruminants : — " The great difficulty which all 

 zoologists have felt in subdividing them into natural minor groups 

 arises from the fact that the changes in different organs (feet, skull 

 frontal appendages, teeth, cutaneous glands, &c.) have proceeded with 

 such apparent irregularity and absence of correlation, that the different 

 modifications of these parts are most variously combined in different 

 members of the group." 



Such facts as these are apt to have a disheartening effect on the 

 student of morphology. If we try to analyse the feeling of disap- 

 pointment to which they give rise, we shall find, I think, that it is due 

 to a theory we are accustomed to assume as the base of our speculations 

 on phylogeny, with which such facts are irreconcilable. This theory, 

 rarely explicitly stated, but everywhere postulated, holds that when 

 one large natural group of animals was derived from another (as for 

 instance Amphibia from Pisces), this took place by one species of the 

 lower group acquiring new characters and taking to a new method of 

 life. It is then imagined that all the species of the higher group have 

 been derived from the modification of this single ancestral species. 

 The view suggested, however, by the ever increasing number of cases, 

 in which we are forced to assume a parallel development, is that a 

 complete homology or homogeny, and a homoplasy are only after all 

 extreme terms in a series, in which the successive terms are very 

 closely related to each other. We seem driven to the conclusion that 

 when a large natural group of animals was being evolved, the changed 

 environmental conditions, which were causing the evolutional progress, 

 acted not merely on one species but on species belonging to the 

 same or different genera, families, or even orders, and induced similar 

 modifications in them. The course of evolution, therefore, instead of 

 being represented by a single trunk of a tree repeatedly branching — 

 the typical form of the Haeckelian genealogical tree — ought rather to 

 be pictured as a column of parallel stems with interlacing branches, 

 like the stipe of a mushroom. 



The study of systematic zoology in fact suggests, that, given a 

 definite set of environmental conditions, any species having a given 

 general structure exposed to them, will undergo the same change. 



