Many species, well adapted for the resistance of external influences, have survived from very early times to the present day. In the Carboniferous period birds and mammals did not yet exist, and the thick forests, with undergrowths of gigantic ferns, were inhabited by large numbers of articulated animals, amongst which were scorpions and insects. The scorpions of that time resemble in every way those that actually live at the present day in tropical countries; and amongst the insects of that early epoch were some very like the cockroaches of to-day. Certain tree-like ferns of the present time are also very similar to those of the coal period. Amongst the animals the bodies of which are protected by a shell, such as foraminifera and mollusca, certain species have survived even from an earlier time than the coal period. In contrast with this extraordinary survival, there are instances of the complete disappearance of numbers of species of animals and plants. In early times, during the Tertiary epoch, the virgin forests of Europe were inhabited by a large number of monkeys, of which fossil remains are now found, especially in Greece. These formerly existed even in Europe, and some anthropoid apes (_Dryopithecus_) have left traces in the tertiary deposits of France.[28] These animals, notwithstanding that their organisation was superior to that of scorpions and cockroaches, have not been able to adapt themselves to the altered conditions of modern Europe. A similar fate has come upon some of the higher mammals, such as the mammoth and the mastodon. These facts do not bear testimony to the prevalent idea that there exists in nature a law of universal progress tending to the production of organisms more and more perfect from the point of view of complexity of structure. It is incontestable that forms higher in the scale of life have developed only after the appearance of lower forms. But it does not follow that development always takes a progressive march. Man is one of the later species that have appeared upon the earth, but there are others of still more recent date. It is very probable that certain species of lice have appeared subsequent to man, particularly the clothes-louse (_Pediculus vestimenti_). Amongst the true parasites which live only in the human body are some that have acquired their specific characters after the appearance of man. Such are certain tape-worms and microbes, such as a species of _gonococcus_. It is therefore amongst parasites and not to man that we must look for the latest products of creation. In nature, then, there is no blind tendency towards progress. Organisms almost innumerable are born every day with variable characters. Those amongst them which are adapted to existing circumstances survive and produce offspring like themselves, but many do not reach maturity, and, living only for a short time, die without leaving issue.