THE ANIMAL ANCESTRY OF MAN 6$ 



developed in the shape of a pouch for the retention of 

 swallowed air. This was located in the throat behind the 

 gills and was richly siipphed with blood vessels. Perhaps 



p.„,»«sv.9<UMRSS^t^RSeHGI 



Fig. 5. Lobe-fmned fish from Devonian of Russia. Restoration of Diplopterus 



by Pander. 



it was derived from one of the earHer gill pouches which 

 had become enlarged for the reception of air swallowed 

 above the surface of the water, rather than for the extraction 

 of the dissolved air from the water passing through the 

 other gill chambers. However that may be, this accessory 

 breathing organ proved to be of incalculable importance 

 to its possessors for it opened up to them the possibility 

 of invading the dry land and finally of disputing its posses- 

 sion with the insects, who had also invaded it from the 

 water but at an earlier date. At the same time some of the 

 lobe-finned fishes acquired exceptionally strong and fleshy 

 fan-shaped paired fins, by means of which their still more 

 highly evolved descendants were enabled to complete 

 their conquest of the dry land. Thus man owes to these 

 ugly-looking denizens of the Palaeozoic swamps two of his 

 most indispensable possessions, namely lungs and limbs, 

 not to mention many other improvements that they ini- 

 tiated, such as the development of checker-like bony 

 centra surrounding the primitive notochord, the arrange- 

 ment of paired bony plates on the roof of the skull, the 

 development of a double shoulder girdle of underlying 

 and surface elements and the production of a pelvis or bony 

 base for the pelvic fins, dividing the musculature of the 

 thorax from that of the tail. 



THE TETRAPODS INVADE THE LAND 



The earliest four-footed vertebrates appear to have 

 sprung from a still undiscovered family of fishes which 



