REPRODUCTION 57 



Highly significant is the fact that so great a degree of diversity can 

 exist within a single class of vertebrates. The fishes and amphibians show 

 this diversity most markedly. Assuming a genetic series from fish to 

 bird and mammal, the evolution of reproduction has not been a direct 

 progress along one straight and narrow path. Instead, the animals within 

 each class, especially the lower, tried (so to speak) a variety of methods. 

 After the many reproductive "experiments" of the lower vertebrates 

 finally emerged two distinct types to which the higher vertebrates fairly 

 closely adhere. Reptiles and birds exhibit one of these types, mam- 

 mals the other. Yet certain distinctive features of these finally emergent 

 types of reproduction are anticipated by some lower vertebrates. The 

 enormous eggs of oviparous sharks and skates, encased in thick shells, 

 resemble eggs of reptiles. Some viviparous sharks produce vascular 

 uterine structures (see page 48) suggestive of the mammal placenta. 

 Certain viviparous Hzards (genus Seps) develop what is practically a 

 placenta. But there can hardly be any direct genetic connection between 

 these structures in sharks and the somewhat similar structures in reptiles 

 or mammals, nor between the "placenta" of a lizard and that of a higher 

 mammal. The exaggerated filamentous gills of the intra-uterine larvae 

 of some viviparous salamanders and the much expanded bell-shaped gills 

 of the larvae of the marsupial frog, Gastrotheca, suggest that the larva 

 may obtain nutriment as well as oxygen from neighboring maternal sources 

 — potentially a "branchial placenta." 



The marsupial structures of vertebrates afford another example of 

 convergence in evolution — that is, the independent origin of functionally 

 similar but genetically unrelated things. Defining a marsupium as a 

 brood-pouch developed on the external surface of the body- wall, there are 

 marsupial fishes (sea-horse; pipe-fish), marsupial frogs and marsupial 

 mammals. 



Viviparity is commonly thought of as something peculiarly mammalian. 

 Yet there are viviparous fishes, amphibians and reptiles. The only 

 vertebrate class which contains no viviparous members is Aves. In view 

 of the fact that all birds and the most primitive mammals that we know 

 are oviparous and the further fact that oviparity predominates among 

 the lower classes of vertebrates, it is highly probable that the earliest 

 vertebrates were oviparous and that the animals which constituted the 

 main trunk of the vertebrate genealogical tree were oviparous. But 

 viviparity has appeared on twigs of various lower branches of the tree as 

 well as at its mammalian top. 



The chordate ancestors of vertebrates must have been small animals 

 and presumably produced small eggs with little yolk. It is likely that 

 primitive vertebrates had small eggs and that large yolk masses have 

 been secondarily acquired. But even within a small group of vertebrates 



