The effects of the struggle to survive, which here prompts a simpler life with resulting degeneracy and there a different life with resulting new developments, are far from being the only causes of morphological obscurations. Fulfilment of certain highly general requirements gives certain common traits to plants of widely divergent classes; and fulfilment of certain other highly general requirements gives certain common traits to animals of widely divergent classes. It was remarked in the first volume (§ 54_f_) that the cardinal distinction between the characters of plants and animals arises from the fact that while the chief food of plants is universally present the food of animals is scattered. Here it has to be added that to utilize the universally distributed food the ordinary plant needs the aid of light, and has to acquire structures enabling it to get that aid; while the ordinary animal, to utilize the scattered food, must acquire the structures needful for locomotion. Let us contemplate separately the traits hence resulting in the vegetal world and the traits hence resulting in the animal world. The familiar plantain meets the requirement by growing stiff leaves enabling it to press down the competing grasses around which would else shade it; but the great majority of ordinary plants meet the requirement by raising themselves into the air. Hence the need for a stem, and hence the fact that plants of widely unlike natures similarly form stems which, in achieving strength enough to support the foliage and resist the wind, acquire certain adaptive structures having a general similarity. Here from the edge of a pool is a reed, and here from the adjacent copse is a hemlock: the one having grown tall in escaping the shade of its companions and the other in escaping the shade of the surrounding brushwood. On being cut across each discloses a tube, and each exhibits septa dividing this tube into chambers. In either case by the tubular structure is gained the greatest strength with the least material; but there is no morphological kinship between the tubes nor between the septa. Still more marked is the simulation of homology by analogy in another plant which the adjacent ditch may furnish--the common Horsetail. In this, again, we see an elongated vertical-growing part, raising the foliage into the air; and, as before, this is tubular and divided by septa. A type utterly alien from the other two has, by survival of the fittest, been similarly moulded to meet mechanical needs.