202 THE COCKROACH : 



2. Frogs and Toads, unlike other fluviatile animals, develop 

 with metamorphosis. The last and most conspicuous change, 

 however, from the gill-bearing and tailed tadpole to the air- 

 breathing and tailless frog, hardly belongs to the ordinary 

 period of embryonic development. When the tadpole has four 

 limbs and a long tail it has already reached the point at which 

 the more primitive Amphibia (Menopoma, Proteus, &c.) become 

 sexually mature. The loss of the tail, the lengthening of the 

 hind limbs, and the complete adaptation to pulmonary respira- 

 tion, relate to the mode of dispersal of the adult. Cut off from 

 early dispersal by the isolation of their breeding-places and the 

 difficulty of land migration, Frogs migrate from pool to pool as 

 full-grown animals. The eggs are thus laid in new sites, and 

 very small basins ditches and pools which dry up in summer 

 can be used for spawning. To this peculiar facility in finding 

 new spawning grounds the Anura no doubt owe their success in 

 life, of which the vast number of nearly-allied species furnishes 

 an incontrovertible proof. But the adaptation to terrestrial 

 locomotion necessarily comes late in life, after the normal and 

 primitive adult Amphibian condition has been attained. It is 

 by a secondary adult metamorphosis that the aquatic tadpole 

 turns into the land-traversing frog. The change is not fairly 

 comparable to any process of development by which other 

 animals gain the adult structure characteristic of their class 

 and order, but (in respect of the time of its occurrence) 

 resembles the late assumption of secondary sexual characters, 

 such as the antlers of the stag, or the train of the 

 peacock. 



3. Lastly, we come to the exceptional case of Insects which, 

 unlike other terrestrial animals, develop with metamorphosis. 

 The Anurous Amphibia have prepared us to recognise this too 

 as a case of secondary adult (post-embryonic) metamorphosis. 

 Thysanuran or Orthopterous larva) cannot differ very widely 

 from the adult form of primitive Insects. From wingless, 

 hexapod Insects, like Cockroach larvae in all essentials of 

 external form, have been derived, on the one hand, the winged 

 imago, adapted in the more specialised orders to a brief pairing 

 season exclusively spent in migration and propagation ; on the 

 other hand, the footless maggot or quiescent pupa. 



