I agree with my son’s argument and not with rejoinder.  The cause of our difference, I think, is that I look at the number of offspring as an important element (all circumstances remaining the same) in keeping up the average number of individuals within any area.  I do not believe that the amount of food by any means is the sole determining cause of number.  Lessened fertility is equivalent to a new source of destruction.  I believe if in one district a species produce from any cause fewer young, the deficiency would be supplied from surrounding districts.  This applies to your par. 5.  If the species produced fewer young from any cause in every district, it would become extinct unless its fertility were augmented through Natural Selection (see H. Spencer).

I demur to the probability and almost to the possibility of par. 1, as you start with two forms, within the same area, which are not mutually sterile, and which yet have supplanted the parent-form (par. 6).  I know of no ghost of a fact supporting belief that disinclination to cross accompanies sterility.  It cannot hold with plants, or the lower fixed aquatic animals.  I saw clearly what an immense aid this would be, but gave it up.  Disinclination to cross seems to have been independently acquired, probably by Natural Selection; and I do not see why it would not have sufficed to have prevented incipient species from blending to have simply increased sexual disinclination to cross.

Par. 11:  I demur to a certain extent to amount of sterility and structural dissimilarity necessarily going together, except indirectly and by no means strictly.  Look at the case of pigeons, fowls, and cabbages.

I overlooked the advantage of the half-sterility of reciprocal crosses; yet, perhaps from novelty, I do not feel inclined to admit the probability of Natural Selection having done its work so clearly.

I will not discuss the second case of utter sterility; but your assumptions in par. 13 seem to me much too complicated.  I cannot believe so universal an attribute as utter sterility between remote species was acquired in so complex a manner.  I do not agree with your rejoinder on grafting; I fully admit that it is not so closely restricted as crossing; but this does not seem to me to weaken the case as one of analogy.  The incapacity of grafting is likewise an invariable attribute of plants sufficiently remote from each other, and sometimes of plants pretty closely allied.

The difficulty of increasing the sterility, through Natural Selection, of two already sterile species seems to me best brought home by considering an actual case.  The cowslip and primrose are moderately sterile, yet occasionally produce hybrids:  now these hybrids, two or three or a dozen in a whole parish, occupy ground which might have been occupied by either pure species, and no doubt the latter suffer to this small extent.  But can you conceive that any individual plants of the primrose and cowslip, which happened to be mutually rather more sterile (i.e. which when crossed yielded a few less seeds) than usual, would profit to such a degree as to increase in number to the ultimate exclusion of the present primrose and cowslip?  I cannot.