While the writer does not decline to accept Ehrlich’s interpretation of the arsenic-fast strains of trypanosomes or Kammerer’s statements in regard to the inheritance of acquired character, he feels that more work should be done before they can be used for our problem. 7. This attitude leaves us in a quandary. The whole animated world is seemingly a symphony of adaptation. We have mentioned already the eye with its refractive media so well curved and placed that a more or less perfect image of the outside objects is focussed exactly on the retina; and this in spite of the fact that lens and retina develop independently; we have mentioned and discussed the cases of instincts or automatic arrangements which are required to perpetuate life--the attraction of the two sexes and the automatic mechanisms by which sperm and egg are brought together; the maternal instincts by which the young are taken care of; and all those adaptations by which animals get their food and the suitable conditions of preservation. Can we understand all these adaptations, without a belief in the heredity of acquired characters? As a matter of fact the tenacity with which some authors cling to such a belief is dictated by the idea that this is the only alternative to the supra-naturalistic or vitalistic ideas. The writer is of the opinion that we do not need to depend upon the assumption of the heredity of acquired characters, but that physiological chemistry is adequate for this purpose. The earlier writers explained the growth of the legs in the tadpole of the frog or toad as a case of an adaptation to life on land. We know through Gudernatsch that the growth of the legs can be produced at any time even in the youngest tadpole, which is unable to live on the land, by feeding the animal with the thyroid gland. As we have stated in Chapter VII, it is quite possible that in nature the legs of the tadpole begin to grow when enough of the thyroid or a similar compound has been formed or is circulating in the animal. It might justly be claimed as a case of adaptation that the egg attaches itself to the wall of the uterus and calls forth the formation of the decidua. We have mentioned the observation of Leo Loeb that the corpus luteum of the ovary gives off a substance to the blood which alters the tissues in the uterus in such a way that contact with any foreign body (_e. g._, the egg) induces this decidua formation. Again what appeared as adaptation when unknown turns out to be a result of the action of a definite chemical substance circulating in the body. It appears as a case of adaptation that the eggs of the majority of animals cannot develop without a spermatozoön, and yet we can imitate the activating effect of a spermatozoön on the egg by definite chemical compounds, which leads to the suggestion that the activating effect of the spermatozoön on the egg might be due to the fact that it carries such a compound.