REPRODUCTION 37 



after generation the life of the world goes on with at most only very slow 

 change in the general biological balance and scheme of things. 



Fishes and amphibians show this reproductive diversity most mark- 

 edly. 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, have tried (so to speak) a variety of methods. From the many 

 reproductive "experiments" of the lower vertebrates finally emerge two 

 distinct types to which the higher vertebrates fairly closely adhere. 

 Reptiles and birds exhibit one of these types, mammals the other. Yet 

 certain distinctive features of these finally emergent types of reproduction 

 are anticipated by some lower vertebrates. The enormous eggs of ovip- 

 arous sharks and skates, encased in thick shells, resemble eggs of 

 reptiles. Some viviparous sharks produce vascular uterine structures 

 (see page 32) suggestive of the mammalian placenta. Certain vivi- 

 parous lizards (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 — prac- 

 tically 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 mam- 

 malian. 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 con- 

 stituted 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 



