28 MAMMALIAN DESCENT. [LECT. II. 



active lifetime, very far above its first condition, put be- 

 fore your eyes the actual transformation of the Common 

 Frog, with the metamorphosis of which every one is 

 familiar. Having performed this mental feat, you will 

 have brought up from the " vasty deep " a type that, 

 in its larval state, would, probably, be intermediate be- 

 tween the Lancelet (Amphioxus), and the Tadpole, or 

 larva of the Frog. Now the Lancelet has no brain, no 

 skull, slight dimensions, and scarcely any sense-organs ; 

 it is, in reality, a sort of half-way creature between a 

 larval Sea-squid (Ascidian), and the lowest of the Verte- 

 brata. But the Tadpole, or larval Frog, represents a 

 low ancient kind of sucking fish (a sort of Lamprey) ; it 

 has a brain, a skull, two sorts of gills, and soon shoots 

 up into a musical, agile, air-breathing Frog. 



Once more let your imaginary forces work, and feign 

 one of these ancient double-breathing fish (Dipnoi), 

 formed by transformation from that supposed low type, 

 and you have a stock which will grow you further 

 suckers for your life-tree. 



Such a form or type, richly charged with morphological 

 force, might transform again and again undergo, under 

 the stimulus of necessity, further metamorphoses. For 

 having both outer and inner gills, and a sacculated air- 

 bladder, acting as a rudimentary lung, it might, under the 

 compelling force of threatening surroundings, suddenly 

 blossom out into one of .the root-types of the higher 

 organic forms. 



