This is in reality the old question of the "inheritance of acquired characteristics," or, in a word, of modifications--a question which has been debated heatedly and at length. And as in many similar instances the number of essays and the length and heat of the debate have been inversely as the number and clearness of the pertinent facts. The large majority of biologists have long felt that the great bulk of the evidence was on one side, namely, that acquired traits were not heritable. At the same time they have recognized the difficulty of explaining certain apparently demonstrated contradictory facts. Some recent experimental work has largely cleared away the theoretical difficulties in this field, and the present status of the old and really fundamental question may be stated as follows: External conditions--climate, temperature, moisture, nutritional conditions, results of unusual activity, and the like--incidences of the environment, undoubtedly produce effects upon the structure and behavior of the organism, but these effects must be clearly grouped into two distinct classes. In the first place the effect of "external" conditions may be to bring about a reaction between the _bodily_ parts affected and the environing conditions. Here the body alone is modified and not the germinal substance for the next generation within this body. Such responses to environing conditions do not affect nor involve the structure of the germ, and are therefore unrepresented in that series of reactions that result in the production of an individual of the next generation. In this class are found most of the instances of "functional modification" or acquired characteristics. In this category belong most of the stock illustrations--from the blacksmith's arm and the pianist's fingers, to the giraffe's neck and the fox's cunning. Here also belong the results of training and education; we can train and educate brain cells but not germ cells. It is characteristic of most of these bodily reactions to external conditions that they are adaptive; that is, when a body reacts to such a condition it does so by undergoing a change which makes the organism better fitted to the new condition--better able to exist. The increased keenness of vision, the strengthened muscle, the thickened fur--all such changes meet new or unusual demands in such a way that the organism has better chances of survival than it would have had unmodified.