τὸ δ᾿ ἤδη παρῆχθαί τε καὶ προστίθεσθαι τῷ μορίῳ τὸ τέλος ἐστὶν αὐτό, δι᾿ ὃ καὶ τῆς τοιαύτης ἐνεργείας ἐδεήθημεν· ἵνα γὰρ προστεθῇ, διὰ τοῦθ᾿ ἕλκεται. χρόνου δ᾿ ἐντεῦθεν ἤδη πλείονος εἰς τὴν θρέψιν τοῦ ζῴου δεῖ· ἑλχθῆναι μὲν γὰρ καὶ διὰ ταχέων τι δύναται, προσφῦναι δὲ καὶ ἀλλοιωθῆναι καὶ τελέως ὁμοιωθῆναι τῷ τρεφομένῳ καὶ μέρος αὐτοῦ γενέσθαι παραχρῆμα μὲν οὐχ οἷόν τε, χρόνῳ δ᾿ ἂν πλείονι συμβαίνοι καλῶς. ἀλλ᾿ εἰ μὴ μένοι κατὰ τὸ μέρος ὁ προστεθεὶς οὗτος χυμός, εἰς ἕτερον δέ τι μεθίσταιτο καὶ παραρρέοι διὰ παντὸς ἀμείβων τε καὶ ὑπαλλάττων 145τὰ χωρία, κατ᾿ οὐδὲν αὐτῶν ǁ οὔτε πρόσφυσις οὔτ᾿ ἐξομοίωσις ἔσται. δεῖ δὲ κἀνταῦθά τινος τῇ φύσει δυνάμεως ἑτέρας εἰς πολυχρόνιον μονὴν τοῦ προστεθέντος τῷ μορίῳ χυμοῦ καὶ ταύτης οὐκ ἔξωθέν ποθεν ἐπιρρεούσης ἀλλ᾿ ἐν αὐτῷ τῷ θρεψομένῳ κατῳκισμένης, ἣν ἀπὸ τῆς ἐνεργείας πάλιν οἱ πρὸ ἡμῶν ἠναγκάσθησαν ὀνομάσαι καθεκτικήν.
Ὁ μὲν δὴ λόγος ἤδη σαφῶς ἐνεδείξατο τὴν ἀνάγκην τῆς γενέσεως τῆς τοιαύτης δυνάμεως καὶ ὅστις ἀκολουθίας σύνεσιν ἔχει, πέπεισται βεβαίως ἐξ ὧν εἴπομεν, ὡς ὑποκειμένου τε καὶ προαποδεδειγμένου τοῦ τεχνικὴν εἶναι τὴν φύσιν καὶ τοῦ ζῴου κηδεμονικὴν ἀναγκαῖον ὑπάρχειν αὐτῇ καὶ τὴν τοιαύτην δύναμιν.
have been finally brought up and presented to the part is the actual end for which we desired such an activity; it is attracted in order that it may be presented. After this, considerable time is needed for the nutrition of the animal; whilst a thing may be even rapidly attracted, on the other hand to become adherent, altered, and entirely assimilated to the part which is being nourished and to become a part of it, cannot take place suddenly, but requires a considerable amount of time. But if the nutritive juice, so presented, does not remain in the part, but withdraws to another one, and keeps flowing away, and constantly changing and shifting its position, neither adhesion nor complete assimilation will take place in any of them. Here too, then, the [animal’s] nature has need of some other faculty for ensuring a prolonged stay of the presented juice at the part, and this not a faculty which comes in from somewhere outside but one which is resident in the part which is to be nourished. This faculty, again, in view of its activity our predecessors were obliged to call retentive.
Thus our argument has clearly shown1 the necessity for the genesis of such a faculty, and whoever has an appreciation of logical sequence must be firmly persuaded from what we have said that, if it be laid down and proved by previous demonstration that Nature is artistic and solicitous for the animal’s welfare, it necessarily follows that she must also possess a faculty of this kind.