We know the successive steps in the growth of the embryo from the time of fertilization to the end of gestation, but how this vital process is effected is not so evident. What we are certain of is that there is a vital principle of some kind from the beginning, and this is the matter of real importance in the present discussion. The old moralists held that this principle in the human being is at first vegetative; after a while that vegetative vital principle is expelled by a sensitive principle; and finally this sensitive soul is expelled by the rational vital principle, or human soul. St. Thomas[40] says: "Some tell us the vital acts that appear in the embyro are not from its soul, but from the soul of the mother, or from the primitive force in the semen. Both these statements are false. Vital operations, as sensation, nutrition, growth, cannot come from an extrinsic principle; therefore it must be admitted that a soul preëxisted in the embryo, nutritive at first, then sensitive, and finally intellectual." After showing that an intellectual soul cannot be evolved from lower forms, he concludes: "Therefore we say that since the generation of one thing is always the corruption of another, in man as in other animals, when a more perfect form comes in this supposes the corruption of any precedent form; so, however, that the sequent form has all perfection that was in the destroyed forms, and something in addition: and thus through many generations and corruptions the final substantial form is attained in man and other animals. This is apparent to the senses in animals generated from putrefaction. Therefore the intellectual soul is created by God at the end of human generation, and this soul is both sensitive and nutritive, all precedent forms having been destroyed." [40] I, q. 118, a. 2, ad 2. There is no such thing as the generation of any animal or other living being from putrefaction; but that is irrelevant. St. Thomas's argument proves conclusively that _if_ man has first a merely vegetative soul, and secondly a merely sensitive soul, which includes the power of the vegetative soul, and thirdly an intellectual soul, which does the work of all three, that this final intellectual soul is not an evolution of the first two, but a new form that replaces these after they have served their purpose and have been annihilated. It does not even attempt to prove that man really has first a merely vegetative soul, and secondly a sensitive, and lastly an intellectual soul; it supposes all this. It starts out with the erroneous Aristotelian theory and takes it for granted. The reason for this statement is that the rational substantial form requires disposed matter to work upon, and the Thomists suppose (again erroneously) that in the human embryo during the period immediately after conception there is not enough matter to be a receptacle for the rational soul.