SELF-FERTILIZATION OF PLANTS. 387 
been grown under “ nearly similar conditions" does not explain it; for Ipomea had been 
so grown; and in making his experiments, Mr. Darwin was most careful to make the 
soil, moisture, light, &c. as absolutely identical as possible for his pairs of plants, so 
that the benefit of crossing should be solely confined to the effect of the sexual process. 
On the other hand, Canna has been always grown in pots; and when no object has been 
in view beyond mere cultivation, it is a small chance that the soil should have been so 
identically the same in every case. The cause cannot then be looked for solely, if at all, 
in environment; it must be in the fact of habitual self-fertilization. It is perhaps worth 
observing that, if a single instance may be trusted, the mean ratio of all the intercrossed 
to the self-fertilized of Mr. Darwin's experiments is exactly the same ratio of the self- 
fertilized to the intercrossed of Pisum sativum, or 100 : 87. 
I do not see, therefore, how we can avoid the conclusion that, if a plant is habitually 
self-fertilizing, it can amply retain its numbers by propagation, and in no way deterio- 
rates in consequence of the process, though, on the other hand, it may derive immediate 
and great benefit from a cross with a new variety or stock. 
However, that a plant may derive no benefit from such an intercross was proved by 
Mr. Darwin in the case of ** Hero," the remarkable self-fertilizing individual of Ipomea, 
of which Mr. Darwin says (l. c. p. 51):— 
* No advantage, as far as we can judge, was derived from intercrossing two of the grandchildren of 
Hero, any more than when two of the children were crossed. It appears, therefore, that Hero and 
its descendants have varied from the common type, not only in acquiring great power of growth 
and increased fertility when subjected to self-fertilisation, but in not profiting from a cross with a 
distinct stock ; and this latter fact, if trustworthy, is a unique case, as far as I have observed in all my 
experiments." 
Mr. Darwin calls this a “ unique ease; " but in Table C, which treats of ratios derived 
from fresh stocks, offspring of Eschscholizia californica, being from Brazil, crossed by an 
English stock, the self-fertilized plant beat the crossed in height and weight, but only fell 
short of it in fertility. But as this plant was self-sterile in Brazil, yet acquired a power 
of self-fertility in England in the ratio of 100:15 in the first year, which was raised to 
100:40 in the second generation (Table C), this seems to show that even in fertility the 
self-fertilized plants were rapidly gaining upon those crossed with a new stock, and 
would equal them, at the same rate of increase, in very few generations. 
Such exceptional cases as the above clearly prove that it can by no means be regarded 
as an absolute fact that intercrossing plants of the same stock, or crossing plants of 
different stocks, does necessarily benefit them. The process of crossing is, as Mr. Darwin 
clearly proves, solely a means to an end, that end being the introduction of new consti- 
tutional elements; and if a cross cannot do this, then the plant, so far from being bene- 
fited, is as much “ deteriorated " as a plant which is habitually intercrossed is “ impaired ” 
by self-fertilization. ** Deterioration” or “injuriousness” are only relative terms; for 
if a plant like Pisum sativum habitually fertilizes itself, and its average height be 
represented by 100, and if, by intercrossing different plants, that standard instantly 
becomes lowered to 87, we are as much justified in saying that the plant was “ deterio- 
“rated” by the cross as, on the other hand, that Zpomea is “ benefited " to the same 
v extent z2 aa ob Ces is true for the 4 one : must be logically true for the other. 
