H 4 CHAPTERS. ON EVOLUTION. 



ments of a single long and continuous fold existing in each side of the 

 young fish ; and the unpaired fins arise from folds of like nature. Thus, 

 if the history of the individual may again be held to explain the evolu- 

 tion of the race, then we may conceive of the first limbs having been 

 developed as a pair of long and unbroken side-fins, which ultimately 

 became detached or broken up to form the paired fins as we see 

 these organs in the fishes of to-day. When the simplest types of 

 limbs in fishes are examined, as for example in the Ceratodus or 

 " Barramunda" of Australian rivers (Fig. 50) the native " salmon" 

 of the colonists the primitive nature of such fins appears to accord 

 well with the idea of their origin and formation as above described. 

 In Ceratodus the skeleton of the limbs (Fig. 50, a, b, and Fig. 51) 

 appears as a simple many-jointed rod of gristle (Fig. 51, a), to 



the sides of which the 

 equally simple fin-rays 

 are attached. It is 

 equally interesting to 

 find that the lowest and 

 presumably the oldest 



F,G.~ 5 i.-F, N OF CERATODUS. and mOSt Pitive fishes 



the lancelet (Pig. 44) 



and the lamprey (Fig. 45) tribe are absolutely destitute of paired 

 fins or limbs. These latter fishes may thus be regarded as presenting 

 us with a representation of those early stages in piscine existence, 

 before the limbs became specialised, and when the unpaired and 

 median fins alone represented the organs of motion. That both 

 pairs of limbs were probably developed from one and the same 

 structure is rendered more than probable when we discover that 

 in some fishes the pectoral and ventral fins exactly resemble each 

 other such a likeness being well seen in the somewhat remark- 

 able fish, allied to the sharks, &c., and known as the Chimcera, or 

 " King of the Herrings." The development and growth of the paired 

 fins or limbs became localised, and thus brought about the separation 

 of the limbs and their distinction from the continuous side-tolds which 

 gave them birth ; whilst the growth of the unpaired fins, on the other 

 hand, continued throughout the entire length of the fold, and resulted 

 in the production of the back, tail, and anal fins as we find them 

 to-day. Amidst much speculation and not a few theoretical con- 

 siderations regarding the nature of the limb-girdles or supports, which 

 it must be left to future research to substantiate or nullify, there still 

 remains to us a large share of true and exact philosophy in what is 

 definitely known regarding the genesis of limbs. In such a study we 

 discern a new phase of the ever-recurring watchwords of the evolu- 

 tionist, "modification" and "descent." And we are led also to 

 note that in the past history of even the most familiar structures of 



