EVIDENCE FROM TAILS, LIMBS, & LUNGS OF ANIMALS. 107 



appear above that formation, and gradually predominate until, as in 

 the present period, the heterocercal (unequal-tailed) bony fishes are 

 almost limited to a single ganoid genus (Lepidosteus)." 



Not until we pass far into the Mesozoic rocks, and arrive at the 

 Chalk, do we meet with fossil representatives of the familiar fishes 

 (such as our herring, salmon, cod, &c.) which swarm in the seas ot 

 to-day, and which, as we have seen, possess apparently equal tail-fins. 

 After the beginning of the Mesozoic 

 period, we discover that the ganoid 

 and other unequal-tailed fishes begin 

 to decline in numbers, many groups 

 becoming wholly extinct; whilst only 

 a comparatively few representatives 

 of these early fishes remain in our 

 seas of to-day to represent, like 

 "the last of the Mohicans," their 

 plentiful development in the oceans 

 of the past. 



The geological evidence, then, 

 reads very strongly in favour of 

 the evolutionist's views concerning 

 the great antiquity of the unequal- 

 tailed fishes. We may see, theoreti- 

 cally, the first beginnings of the 

 fish-tail paralleled by the first stage of the modern flounder (Fig. 43, A), 

 and by the permanent condition of the living lancelet and lampreys ; 

 presenting us with a symmetrical end to the body, but with no very char- 

 acteristic or definite tail. Next in order in point of time, come the 

 ganoid fishes (Fig. 46), and the representatives of the sharks (Fig. 41), 

 skates, and rays, with tails of the truly unequal conformation, the spine 

 bending upwards into the upper half of the tail an era in the 

 development of the fish group represented by the second stage ot 

 the flounder (Fig. 43, B), when the extremity of the back-bone is seen 

 to undergo a similar alteration in growth. Ultimately we attain in 

 the Chalk to the modern order of things, and find therein the first 

 appearances of fish-tails of the modern and equal type a conforma- 

 tion which, as we have seen, really retains, under the guise of an 

 outward symmetry, the evidence (Figs. 42 and 43, c) of its connection 

 with the unequal tail of long ago. Thus perfectly does the geological 

 evidence harmonise with that of development, in showing us how 

 modification and evolution have represented the laws of fish-produc- 

 tion. It is only needful, by way of close to such a history, to remark that 

 the laws of evolution and of the production of fishes through descent 

 and modification follow, in their uncompromising application alike 

 to higher and lower life, the boasted impartiality of the legal codes 



FIG. 46. PTERICHTHYS, A FOSSIL FISH 

 (OLD RED SANDSTONE). 



