Now we may apply this conception to the sexual combination of two different elementary species, assuming one to be the derivative of the other. The differentiating mark is only present in one of the parents and wanting in the other. While all other units are paired in the hybrid, this one is not. It meets with no mate, and must therefore remain unpaired. The hybrid of two such elementary species is in some way incomplete and unnatural. In the ordinary course of things all individuals derive [254] their qualities from both parents; for each single mark they possess at least two units. Practically but not absolutely equal, these two opponents always work together and give to the offspring a likeness to both parents. No unpaired qualities occur in normal offspring; these constitute the essential features of the hybrids of species and are at the same time the cause of their wide deviations from the ordinary rules. Turning now to the varieties, we likewise need discuss their differentiating marks only. In the negative types, these consist of the apparent loss of some quality which was active in the species. But it was pointed out in our last lecture that such a change is an apparent loss. On a closer inquiry we are led to the assumption of a latent or dormant state. The presumably lost characters have not absolutely, or at least not permanently disappeared. They show their presence by some slight indication of the quality they represent, or by occasional reversions. They are not wanting, but only latent. Basing our discussion concerning the process of crossing on this conception, and still limiting the discussion to one differentiating mark, we come to the inference, that this mark is present and active in the species, and present but dormant in the variety. Thus it is present in both, and as all other characters not differentiating [255] find their mates in the cross, so these two will also meet one another. They will unite just as well as though they were both active or both dormant. For essentially they are the same, only differing in their degree of activity. From this we can infer, that in the crossing of varieties, no unpaired remainder is left, all units combining in pairs exactly as in ordinary fertilization. Setting aside the contrast between activity and latency in this single pair, the procedure in the inter-crossing of varieties is the same as in ordinary normal fertilization. Summarizing this discussion we may conclude that in normal fertilization and in the inter-crossing of varieties all characters are paired, while in crosses between elementary species the differentiating marks are not mated.