1897- MORPHOLOGY IN ZOOLOGICAL SCIENCE. 25 



islands, or in small lakes may have allowed them to do so ; but in the 

 case of what we may term phylum-ancestors such as Aniphioxus and 

 Pevipatiis such explanations are futile. The grade of structure they 

 present, dates from such a remote epoch that no physical barrier will 

 have remained constant in the interval, added to which their wide 

 distribution at once negatives such an idea. No ; the general method 

 in which such animals escaped from the stress of environment, was 

 by taking to burrowing or sessile modes of Ufe, and this has 

 inevitably carried a certain amount of degeneracy with it. The idea 

 that active vigorous vertebrates are descended fnom a sluggish mud- 

 eating worm like Balanoglossus must strike many people as highly 

 improbable, but that Balanoglossus is descended from a free swimming 

 form with well developed eyes we know from the structure of its larva 

 to be extremely probable, and there is nothing violent in the supposi- 

 tion that this same form may have given rise to the Vertebrata. I 

 think, if once the dictum, " All primitive animals are also in some 

 degree degenerate " were accepted, a harmonious explanation of many 

 discordant facts would be attained. 



In this connection I may say that such phrases as " degree of 

 modification " are in need of definition. I think many zoologists have 

 admitted there are two kinds of modification at work : first, in the 

 intensity of metabohsm or the degree of vitaHty, by which we mean 

 the degree of specialisation of the organs for carrying on the vital 

 processes — digestion, respiration, circulation, excretion, &c. ; and 

 secondly, in shape and form of the outer appendages, or of the 

 general shape of animals. Now, it is the first kind of modification to 

 which zoologists attach weight. What we desire above all to know 

 is how the complicated physiological mechanisms represented by the 

 higher vertebrates have been built up. It is assumed that variations 

 in form and size— the outgrowth of flaps and lobes, &c. — are things 

 which may have been accompHshed in a comparatively short time. 

 When, therefore, zoologists speak of animals being primitive, they 

 mean with regard to internal organisation. On grounds of the 

 relatively greater importance of the latter, they separate the beaver 

 and wombat, the great ant-eater and the pangolin, &c., &c. This 

 distinction between internal and external structures is sometimes 

 explained by saying that the latter are adaptive, as if all characters 

 were not ultimately adaptive. Explicitly stated, the belief underlying 

 such principles of classification is : Improvement and elaboration of 

 internal structure and constitution is a slow process ; modification 

 of external structure is a rapid one, and has given similar results 

 again and again. The Brachiopoda are a good instance of a group 

 where the external form is highly speciahsed, but where inner differ- 

 entiation is at a very low ebb. They exhibit a most primitive 

 condition of the excretory, generative, circulatory, and nervous 

 organs. These animals form a part of the oldest fauna known to us ; 

 but if the view which I have been trying to propound is correct, the 



