The same causes must be held responsible for the following groups of facts.(1) Some children resemble their parents, while others do not; some being like the father and others like the mother, both in the body as a whole and in each part, male and female offspring resembling father and mother respectively rather than the other way about.(2) They resemble their parents more than remoter ancestors, and resemble those ancestors more than any chance individual.(3)Some, though resembling none of their relations, yet do at any rate resemble a human being, but others are not even like a human being but a monstrosity.For even he who does not resemble his parents is already in a certain sense a monstrosity; for in these cases Nature has in a way departed from the type.The first departure indeed is that the offspring should become female instead of male; this, however, is a natural necessity.(For the class of animals divided into sexes must be preserved, and as it is possible for the male sometimes not to prevail over the female in the mixture of the two elements, either through youth or age or some other such cause, it is necessary that animals should produce female young).And the monstrosity, though not necessary in regard of a final cause and an end, yet is necessary accidentally.As for the origin of it, we must look at it in this way.If the generative secretion in the catamenia is properly concocted, the movement imparted by the male will make the form of the embryo in the likeness of itself.(Whether we say that it is the semen or this movement that makes each of the parts grow, makes no difference; nor again whether we say that it 'makes them grow' or 'forms them from the beginning', for the formula of the movement is the same in either case.) Thus if this movement prevail, it will make the embryo male and not female, like the father and not like the mother; if it prevail not, the embryo is deficient in that faculty in which it has not prevailed.By 'each faculty' I mean this.That which generates is not only male but also a particular male, e.g.
Coriscus or Socrates, and it is not only Coriscus but also a man.In this way some of the characteristics of the father are more near to him, others more remote from him considered simply as a parent and not in reference to his accidental qualities (as for instance if the parent is a scholar or the neighbour of some particular person).
Now the peculiar and individual has always more force in generation than the more general and wider characteristics.Coriscus is both a man and an animal, but his manhood is nearer to his individual existence than is his animalhood.In generation both the individual and the class are operative, but the individual is the more so of the two, for this is the only true existence.And the offspring is produced indeed of a certain quality, but also as an individual, and this latter is the true existence.Therefore it is from the forces of all such existences that the efficient movements come which exist in the semen; potentially from remoter ancestors but in a higher degree and more nearly from the individual (and by the individual Imean e.g.Coriscus or Socrates).Now since everything changes not into anything haphazard but into its opposite, therefore also that which is not prevailed over in generation must change and become the opposite, in respect of that particular force in which the paternal and efficient or moving element has not prevailed.If then it has not prevailed in so far as it is male, the offspring becomes female;if in so far as it is Coriscus or Socrates, the offspring does not resemble the father but the mother.For as 'father' and 'mother' are opposed as general terms, so also the individual father is opposed to the individual mother.The like applies also to the forces that come next in order, for the offspring always changes rather into the likeness of the nearer ancestor than the more remote, both in the paternal and in the maternal line.
Some of the movements exist in the semen actually, others potentially; actually, those of the father and the general type, as man and animal; potentially those of the female and the remoter ancestors.Thus the male and efficient principle, if it lose its own nature, changes to its opposites, but the movements which form the embryo change into those nearly connected with them; for instance, if the movement of the male parent be resolved, it changes by a very slight difference into that of his father, and in the next instance into that of his grandfather; and in this way not only in the male but also in the female line the movement of the female parent changes into that of her mother, and, if not into this, then into that of her grandmother; and similarly also with the more remote ancestors.
Naturally then it is most likely that the characteristics of 'male' and of the individual father will go together, whether they prevail or are prevailed over.For the difference between them is small so that there is no difficulty in both concurring, for Socrates is an individual man with certain characters.Hence for the most part the male offspring resemble the father, and the female the mother.For in the latter case the loss of both characters takes place at once, and the change is into the two opposites; now is opposed to male, and the individual mother to the individual father.
But if the movement coming from the male principle prevails while that coming from the individual Socrates does not, or vice versa, then the result is that male children are produced resembling the mother and female children resembling the father.