SERVICE IN EVOLUTION 193
mid-Paleozoic time the forced yielding of the parent's life for the pro- duction of young. There were probably many protozoons of which this was tme, as it nndonbtedly was of many corals whose remains have been preeerved, as well as of the many herb-like plants with a single growing season. It was^ on the whole^ the lower plant or animal groups only whose life span was thns reduced; to the higher groups a longer life was a necessity. Such higher individuals had been evolved by mid and upper Paleozoic time and these were compelled to prepare a consider- able amount of nourishment for the developing embryo. This was true of the primitive seed-plants^ as well as of the early &&h, amphibians and reptiles.
If we may judge by the most nearly related living forms, there must likewise have been present during the entire Paleozoic a rudi- mentary kind of instinct^ with but a minimum amount of free choice and hence of voluntary service. Even the insects, first appearing in the upper Paleozoic and rapidly becoming so numerous in the vast coal- swamps of that time, all belonged to the lower orders with a very low degree of instinct.
With the evolution in the Mesozoic era of the higher seed-plants, insects and fish, and of the most primitive birds and mammals, a higher type of compxdsory service was initiated and a distinct beginning in voluntary service made. The higher seed-plants, typified by the oak and hickory, furnished a larger amount of embryonic nutriment and better seed protection than did the primitive seed-plaixts of the upper Paleo- zoic. The development of bees, ants and wasps with the mid-Mesozoic was most probably accompanied with a beginning of that wonderfully evolved instinct and of some voluntary service to their young, as well as to the members of the community, which characterize their modern representatives. The incoming at the same time of the highest order of fishes, the Teleostei, may likewise have been accompanied in some indi- viduals, as it is ia many of their living descendants, by a certain amount of voluntary service.
Some of the Mesozoic mammals, allied to the existing monotremes, probably like these laid eggs, hatched them as birds now do and then suckled the young; while others, more nearly related to the kangaroos and insectivores were, in all probability, like their modern represent- atives, forced to protect and nourish the embryo within the body until well developed and after birth to continue this care by fighting off enemies and nursing the young. While all the service before birth, and much after it, was compulsoiy there still remained a distinct amount of voluntaiy service both in hatching the eggs and in feeding and protect- ing the offspring.
During the Cenozoic appeared the highest forms of service, both
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