178 PRIMITIVE FORMS OF LIFE 



obliged to return to their earlier habitat, the water, in order 

 to spawn. Indeed, this is a general rule resulting from the 

 fundamental embryogenetic law, according to which the 

 young pass through the same stages of development as their 

 ancestors. As the latter were originally aquatic, they ought to 

 have offspring which begin with an aquatic stage, and then 

 progress more or less rapidly towards a terrestrial existence. 

 We may, indeed, consider it a law that when an animal has 

 changed its environment it returns for a long time after to its 

 original environment in order to reproduce itself. Thus the 

 Land-crabs return to the water in vast hordes to spawn there, 

 while the Seals leave the water to give birth to their young 

 on land. Such reversion, nevertheless, ceases in the end ; 

 the Whales, for instance, give birth to their young in the water. 

 The necessity for a periodical return to the water would 

 have inevitably retained the Batrachians, which are poor 

 walkers, close to the shores, and would have interfered with 

 the peopling of land. Fortunately, tachygenesis removed the 

 obstacle. Its action has been exercised in different ways. The 

 Ccecilians, Batrachians without tails or limbs, which live below 

 ground by burrowing in the earth like the worms they so closely 

 resemble, lay their eggs in their subterranean galleries without 

 returning to the water. None the less, in their embryonic 

 stage the young have enormous racquet-shaped or ramified 

 branchiae, completely enveloping them, which serve them 

 for respiration as well as for protection, and are finally 

 absorbed before birth. The young terrestrial Salamanders have 

 almost completed their metamorphosis when they are born in 

 the water, and the closely related black Salamanders of the 

 Southern European mountains no longer return to the water 

 to lay their eggs, but are now viviparous. Under favourable 

 conditions, viviparity can be induced in the land Salamanders 

 themselves. The young Pipa is likewise hatched at an advanced 

 stage of development, in the hollow pustules on the back of the 

 male, where the eggs have been lodged, the male remaining 

 in the water during the period of incubation. Certain Anura 

 (Leptodactylus ocellatus and L. mystacinus, Paludicola gracilis, 

 Pseudophryne australis and P. briboni) lay their eggs out of the 

 water, whither the young are brought by heavy rains after they 

 are hatched. The Chiromantis and Phyllomedusa attach their 

 eggs, enveloped in a glairy substance to the underside of the 



