[Illustration:  FIG. 22.—­DIAGRAMMATIC REPRESENTATION OF THE PROCESS OF FERTILIZATION. (Boveri.) In the first cell (a) the ovum is shown in process of fertilization by the entering spermatozoon or male sexual element.  In the following cells there is shown the increase in amount of the male material and the final intimate commingling in g which precedes the first segmentation. g represents a new organism formed by the union of the male or female cell but differing from either of them.]

Living matter is always individual, and this individuality is expressed in slight structural variations from the type of the species as shown in an average of measurements, and also in slight variations in function or the reactions which living tissue shows towards the conditions acting upon it.  The anatomical variations are more striking because they can be demonstrated by weight and measure, but the functional variations are equally numerous.  Thus, no two brains react in exactly the same way to the impressions received by the sense organs; there are differences in muscular action, differences in digestion; these variations in function are due to variations in the structure of living material which are too minute for our comparatively coarse methods of detection.  In the enormous complexity of living matter it is impossible that there should not be minute differences in molecular arrangement and to this such functional variations may be due.  Chemistry gives us a number of examples of variations in the reaction of substances which with the same composition differ in the molecular arrangement.  Even in so simple a mechanism as a watch there are slight differences in structure which gives to each watch certain individual characteristics, but the type as an instrument constructed for recording time remains.  In the fusion of the chromosomes of the male and female sexual cells, to which the hereditary transmission of the ancestral qualities to the new offspring is due, there are differences in the qualities of each, for the individuality of the parents is expressed in the germ cells, and the varying way in which these may fuse gives to the new cell qualities of its own in addition to qualities which come from each ancestor, and from remote ancestors through these.  The qualities with which the new organism starts are those which it has received from its ancestors plus its individuality.  The fact that the sexual cells are formed from the early formed cells of the new organism which represent all of the qualities of the fertilized ovum or primordial cell, renders it unlikely that the new offspring will contain qualities which the parents have acquired.  The question of the inheritance of characteristics which the parents have acquired as the result of the action of environment upon them is one which is still actively investigated by the students of heredity, but the weight of evidence is opposed to this belief.

In the new organism the type of the species is preserved and the variations from the mean to which individuality is due are slight.  We are accustomed to regard as variations somewhat greater departures from the species type than is represented in individuality, but there is no sharp dividing line between them.