Whether growth is necessary for living things.
At what age is there more growth?
Why in the human, when he is fully grown, are the upper parts smaller than the lower ones?
One inquires now into the growth of living things. First, whether growth is necessary for living things.
1. It seems not because the operation of a nobler form is itself nobler. But the form of something living is nobler than the form of something not living. But a non-living thing, generating by means of its mediating form, simultaneously bestows being and perfected or completed size. This is clear from fire, and is therefore all the more clear in a living thing that generates.
2. Likewise, the form of a non-living thing is nearer to matter than is the form of a living thing. If then the form of the non-living receives being and perfected size at the same time, so much the more does the form of the living do so.
Second, one asks at what age more growth occurs.
1. And it seems that more growth occurs during adolescence than in childhood. Growth is a movement from incomplete to complete. Therefore, the more something approaches full growth [completum] and its natural end, the more it grows, etc.
2. Furthermore, natural motion is directed to an end and is weakened in the beginning, although violent motion is the opposite. But growth is a natural motion, and is therefore directed to an end so that it can grow; therefore, etc. Now adolescents are nearer to the end of that which completes them than are children; therefore, etc.
The Philosopher says the opposite in this book. For he says that a person grows more in the first seven years than in the second, and more in the second seven years than in the third, and grows less in the fourth seven years because then an equilibrium is achieved.
Third, one asks why in the human, when he is fully grown, the upper parts are smaller than the lower parts whereas, in the beginning, it is the other way around and they are larger. This is the opposite of what happens in all the other animals. Now it seems that the upper parts are larger in all cases, because according to the Philosopher the upper parts extend from the head to the bottom of the belly, and the lower parts extend from there to the feet, and the head with the trunk is larger than the feet.
To the first, one should say that growth is necessary to a living thing because it is impossible that a part should be equal to the whole at the same time in the same subject. But living things are generated from seed, which is a part cut off from the one generating. Which is why it is impossible that an animal or a living thing generated from a seed should be equal in the beginning to the one doing the generating. And for that reason living things have three powers: the nutritive, through which it is conserved in its acquired being; the augmentative, through which it proceeds, with nourishment as its mediating agent, from the imperfect to the perfect; and the third is the generative power, through which it produces one like itself.
1. To the first argument one should reply that in non-living things the matter of the one generated did not exist earlier in the one generating, but rather in its opposite. Thus, the one generating introduces form in the matter that is externally determined with respect to its quantity. But in living things the matter of the one generated did exist earlier in the one doing the generating. And this same matter is augmented by the reception of nourishment, and for this reason the form is introduced earlier under one quantity, and then the quantity and the augmented matter are increased. This is why this is attributed to the nobility of the form itself and not to its ignobility.
2. To the second, one must say that non-living things are so near to matter that they have a determinate matter, as is evident in a rock, and for that reason it receives being and a perfected quantity at the same time. But animated things do not receive their entire matter determined at one and the same time, and for that reason they do not receive a perfected quantity [full-grown size] all at once.
To the second inquiry one must reply that two things are required for growth, namely, an agent and one that is acted on. The augmentative power requires heat that digests the food, and requires a nutrimental moisture through which growth might occur. Therefore, more growth occurs at the age when these things are more abundant. But this is during childhood, although sometimes it seems to the senses that there is more growth during adolescence, unless the natural heat is weakened by excessive moisture during childhood. Therefore, as long as an animal can convert nourishment into its own substance, so long as it is sufficient to replace what is lost and more, just so long will it grow. And when it can only convert an amount sufficient to replace what is lost, then it is in stasis, and when it can no longer do this, then it is a time of decreasing size.
1. To the first argument one should reply that the more a living thing approaches full growth, so much more will it grow, in order to achieve larger size, yet in the near term it does not proportionally acquire more size de novo than it had acquired in an earlier period, because the more it approaches full growth, the more its power is weakened owing to the reaction of contraries.
2. To the second argument one must reply that a natural motion is of two types. One is according to its approach to its natural and proper end and occurs with the strengthening of its cause, and such is the motion of the light and the heavy, which inclines to an end. The other is a natural motion resulting from a withdrawing from its cause and accompanied by the weakening of the cause, and such is the motion of growth. And it is weakened for the same reason as the illumination of a medium, for the further something is from a luminous body, like the sun or a candle, the weaker it is on account of the distance from the cause.
To the other, the third, question one must reply that a woman's womb comes near to being round, and for that reason long members, like the tibia, thigh, and leg bones, are not located as conveniently as a round member, like the head and the trunk, and for that reason the head and the trunk are proportionally larger in the womb than smaller. But in other animals the wombs are more stretched out, and for that reason their long members enjoy more solidity. Thus the human, in the beginning of his life, has upper parts that are broad and large in comparison to the lower ones, but as his age increases the upper parts become dried out and as a result they are proportionally reduced.
One must reply to the argument that in every animal the upper parts are larger in size, but are not proportionally larger, and for the reason that, etc.
-  See Ar., HA 2.1 (500a26f.). Recall that the term "feet" can include the entire lower body. As A. points out in DA 1.496, he uses pes, "foot," for the Arabic rigil, the entire lower walking assemblage. Cf. DA 126.96.36.199 (SZ 1: 87).