The Road Ahead

Abstract

All mammals, including man, undergo considerable development inside the uterus. This is a great advantage to them, for it provides their young with almost complete protection until they are born, by which time some of them can almost fend for themselves. Protection of a similar but less satisfactory kind has been achieved in the course of evolution by a number of different phyla, in a number of different ways. Each of these has presented its own problem of how the developing organism shall be nourished, and of how provision can be made for its excretions before it is capable of taking over these functions, both of which are essential for its well being. The production of an egg with a built-in food supply and some protection against the environment is probably the best known way this has been done, but there are others. Some fish and reptiles, the British adder for example, retain the fertilized egg in the oviduct till the young are hatched and well enough developed to fend reasonably well for themselves, and the marsupials have developed a system of premature birth and the provision of an external pouch on the mother’s abdomen in which the young can be fed and protected. Only the mammals, however, have succeeded in bringing the development of their offspring before birth to the pitch of perfection we know it today. The young of some of them, born in an immature state, depend for their survival on the protection of a nest and the warmth provided by their mother.