REPRODUCTION 55 



weeks, but the young kangaroo is carried in the pouch and nourished by 

 mammary glands for about eight months. 



In placental mammals, as compared to marsupials, the young are born 

 at a relatively advanced stage of development and growth. The mam- 

 mary organs, however, are in all cases an important post-natal provision 

 for bringing the young animal along to a degree of size and strength favora- 

 ble to ultimate success. They afford the great advantage, too, that the 

 young animal is not thrown upon the world abruptly but may acquire 

 independence gradually. 



EVOLUTIONARY SIGNIFICANCE 



Surveying the whole group of vertebrates, the great diversity in the 

 conditions and arrangements attending reproduction is most impressive. 

 It would be difficult to imagine any practicable reproductive expedient or 

 condition which is not exhibited by some animal. There are microscopic 

 eggs and there are ostrich eggs. The quantity of egg yolk may be vast 

 or it may be next to nothing. The primary food supply, yolk, may in 

 various ways be supplemented by secondary sources of nutriment — egg 

 albumen, maternal blood, mammary milk, pigeon "milk." One egg or 

 millions of them may be produced at a time. They may or may not have 

 shells. Parental care of eggs or young ranges from nothing to the human 

 maximum. Vertebrates may be oviparous or viviparous. A primary 

 oviparity may be succeeded by a secondary substitute for viviparity, as 

 when eggs develop within a fish's mouth, an amphibian vocal sac, or 

 integumentary pouches of various sorts. Differentiation of organs may 

 precede growth or it may be delayed until the embryo is relatively large. 

 The newly hatched larva of so large a fish as the Atlantic salmon is about 

 0.65 inch long; a new-born whalebone whale is about tw^enty feet long. 

 The embryo may develop directly to the adult form or there may be a 

 larval period terminated by a metamorphosis. The embryo may or may 

 not produce a complex set of temporarily functional membranes — amnion, 

 chorion, allantois. 



The important point to be appreciated is that the association together 

 of any two or more of these various alternatives in a single animal is not 

 haphazard. If one circumstance is, in itself, inadequate for the success 

 of reproduction, it is supplemented by something else. If a large fish 

 were to produce one single microscopic egg annually and deposit it any- 

 where in the Pacific Ocean, the species would soon become extinct. On the 

 other hand, there is no unnecessary duplication of highly specialized 

 arrangements. A placental mammal does not produce a large yolky egg. 

 The entire complex of reproductive conditions occurring in any one 

 animal comprises a consistent grouping of alternatives which, as a whole, 

 is adequate. Despite the great differences in methods of reproduction. 



