II. The idea of species rests, as we have seen, upon the two distinct ideas of _resemblance_ and _filiation_. Let us first turn our attention to the former as being the greater stumbling-block of the two. No one would hesitate to consider two individuals resembling each other very closely as belonging to the same species; if, however, they present somewhat marked differences, and the necessary information is wanting, we hesitate to give our decision in the negative. The mind readily accepts the latter conclusion when man is the object of discussion. A continual, though unconscious study, has endowed us with a perception which appreciates, in those around us, the most delicate gradations in features, the colour of the skin, and in the appearance of the hair. Now this delicacy of appreciation has, in the present instance, a serious inconvenience. It inevitably conduces to the exaggeration of differences existing between different groups, and by this very means leads us to regard them as so many species. For this decision to have a real value, however, it should be shown beforehand that the variations between one human group and another, are greater than those which have been established between groups of animals and plants, which are positively known to be only _races of one species_. Now this is not the case. However slight an attempt we may have made to become acquainted with the nature and the extent of variations, we shall very soon see that in animal and vegetable races they attain limits, which are never overstepped, and but rarely attained, by the differences between human groups. III. I shall not insist at any length upon the morphological and anatomical changes of plants. It will be sufficient to call to mind how numerous and different are those varieties of vegetables, flowers, fruit-trees, and ornamental shrubs, the number of which is always on the increase. Amongst the latter, the variety, it is true, very rarely attains to the condition of a _race_. Grafting, propagation by layers, etc., make it possible to multiply them rapidly and with certainty, as in the case of the thornless acacia, and gardeners have always been in the habit of resorting to this method. Nevertheless, even among fruit-trees, a few of these varieties have become fixed, and can be reproduced by seed. The plum, the peach, and the vine, may be quoted as examples. As to annuals, garden vegetables especially, they can only be preserved and multiplied by this method. Here we only find races, and it is well to know how numerous and varied they are. The cabbage alone (_Brassica oleracea_) numbers forty-seven principal races, each of which is sub-divided into a number of secondary and tertiary races. Now it is quite useless to insist upon the distance which separates the headed cabbage, of which sauerkraut is made, from the turnip-cabbage, of which the root is eaten, and from the cauliflower or the brocoli.