DEVONIAN ANIMALS. 



343 



In Placoids, both living and extinct, there is a similar combination 

 of connecting and embryonic characters, bnt in this case the embry- 

 onic seem to predominate. We have here, as before : 1. The carti- 

 laginous skeleton. 2. The ventral position of the mouth. But in addi- 

 tion to these, also, 3. The absence of an opercle or gill-cover, growing 

 backward and covering the gill-slits. 4. Perhaps the leathery or im- 

 perfectly-rayed condition of the fins; and, 5. The ligamentous instead 

 of bony attachment of the teeth. 



On the other hand, the Placoids, at least those of the present day, 

 have some very high connecting characters in their reproduction. In 

 all Placoids the impregnation is internal and not external, as is usual 

 in Teleosts; and therefore, instead of spawning a great number of 

 small, unimpregnated ovules, they lay either few large, well-covered, 

 impregnated eggs, like birds and reptiles (skates and some sharks), or 

 else incubate their eggs within the oviduct, and bring forth their 

 young alive (ovo-viviparous) like some reptiles. In some cases there is 

 even an attachment between the yolk-sac of the internally-hatched 

 young and the oviduct of the mother, somewhat similar to that of the 

 placenta to the uterus in the mammal. The young of Placoids also 

 have, at first, a kind of external branchiae, like those of amphibians. 



The following schedule briefly embodies these facts. It is seen 

 that in the Ganoids the connecting, in the Placoids embryonic, characters 

 predominate; but that in the Placoids the connecting characters are 



Ganoids. 



Placoids. 



Connecting ■< 

 Embryonic • 



'Bony armor. 

 Teeth labyrinthine. 

 Swim-bladder = lungs. 

 Paired fins = legged. 

 Tail-fin = vertebrated. 

 Skeleton = cartilaginous. 

 Mouth = ventral. 



Reproduction. ) ^ 



Tail-fin = vertebrated. ^Connecting. 



Skeleton = cartilaginous. 



Mouth = ventral. 



Gill-slits uncovered. \- Embryonic. 



Fins imperfectly rayed. 



Teeth imperfectly attached. J 



very high. The Lepido-ganoids of Devonian and Carboniferous times 

 were far more connecting or reptilian than the Ganoids of the present 

 day. Hence these have been called Sauroids by Agassiz and Heiyetich- 

 thyes by Huxley : 



Bearing of these Facts on the Question of Evolution. — It is seen 

 above that the Devonian fishes combined certain high characters with 

 certain low characters. From one point of view they seem lower, from 

 another higher, than ordinary fishes. There has been some dispute, 

 therefore, whether in the history of fishes we find a law of progress or 

 a law of regress ; in other words, whether or not it sustains a law of evo- 

 lution. The dispute is a result of a misconception of the nature of evo- 

 lution. The most fundamental law of evolution is the laiv of differ en- 



