Embryology of certain of the Lower Fishes. 213 



gians and Elasmobranchs flourished exceedingly. The 

 waters were thickly peopled with these vertebrates, which 

 sought refuge by developing a purely aquatic habitat — as 

 others of their contemporaries doubtless did by adopting 

 purely terrestrial habits — from the intense struggle for 

 existence in the shallow marginal zone, where, amidst a 

 more favourable environment, the early forms of life prob- 

 ably arose. In any case, apart from such speculations, the 

 waters certainly had a crowded population of such purely 

 aquatic fishes. But this being so, we must believe that such 

 fishes were intimately adapted to their aquatic mode of life. 

 Now, I have no hesitation in asserting that any one who has 

 spent any considerable time in the study of living specimens 

 of the various types of fishes, will admit that the type such 

 as is seen in the Dipnoan of to-day, as well as in various 

 Teleosts — the type with diphycercal tail and undivided 

 median fin — although well enough adapted to what may be 

 called a bentheic existence, to wriggling about amidst aquatic 

 vegetation, is hopelessly unadapted to an active nectonic or 

 free-swimming existence, and quite unfit to compete with 

 the active type with heterocercal tail and highly specialised 

 segmented median fins. This being so, it would excite no 

 surprise that remains of the protocercal type should be rare 

 or absent in deposits where the nectonic type of fish is 

 abundant ; nor could the rarity or absence of such remains 

 be allowed any weight whatever as evidence that the proto- 

 cercal type did not exist at the time the deposits were laid 

 down. 



Taking an all-round view of the evidence at present avail- 

 able, it is difficult to see how it is in any way sufficient to 

 serve as foundation for such a view as that of Dollo, that 

 the ancestors of the present-day Dipnoans were at any time 

 of the specialised nectonic type. 



Cellular Constitution of the Vertebrate Body. 



One of the Dipnoans which I have dealt with — Lepido- 

 siren — is characterised by the large size of its cell elements, 

 and on this account, taken along with the comparatively 

 lowly organised character of the creature, one is impelled to 



