150 BIOLOGICAL LECTURES: 



as the result of a series of progressive changes, or viewing 

 any fossil form as the highest of its kind up to the time of its 

 existence, unless in either case there is positive proof to the 

 contrary ; for the biogenetic law lays* the burden of proof 

 upon the degenerationist. Life habits, favoring simplification 

 or obliteration of structure, are parasitism, sessile or fixed 

 life, burrowing and hiding in crevices and holes, and indiscrim- 

 inate andftliffuse food habits, while free and unrestricted motion 

 within the aqueous and aerial oceans upon and over their 

 bottoms, involving the pursuit of fleeing prey and the struggle 

 with other forms for its possession and for life itself, are highly 

 favorable to increasing complication of structure. It is at 

 once evident that we cannot take existing vertebrates and 

 group them either in an arbor-like or ladder-like geneological 

 arrangement, and at the same time express the true relation- 

 ships of the many modifications of structure which have been 

 produced. This brings us to the consideration of the question 

 of the classification of Bdellostoma, and to this I shall return 

 after throwing what light I can upon the conditions under 

 which Bdellostoma exists and the probable relations which this 

 fish bears to these conditions of its environments. 



What evidence can we bring forward to show that " the small 

 and degenerate group Cyclostomata" as Lankester * calls them, 

 are not degenerate Vertebrates ? In previous publications I have 

 pointed out some of the facts, and I am sorry to say they have 

 not been accorded that consideration which they merit as 

 anatomical facts. In I88Q 2 I showed that "the higher sense 

 organs of the Cyclostomata are all paired, since the nose (i.e., the 

 nasal or olf active epithelium) exists in the embryo as well as the 

 adult in tJie form of two circumscribed areas lying on either side 

 of the median line, each of which receives the entire nerve supply 

 afforded by the olfactory nerve of its side. I then ventured to 

 predict that the Myxinoids would show the bilaterally sym- 

 metrical or paired nasal areas as distinctly as Petromyzon does. 

 I can now state that my conclusion was well founded, for in 

 Bdellostoma we have the nose divided into right and left halves 



1 Reprint Article, Vertebrates, Ency. Brit. ed. 1887. London, 1890. 



2 Concerning Vertebrate Cephalogenesis, Journal Morphology, IV, 1890. 



