386 REV. GEORGE HENSLOW ON THE 
tion, and the two cases which are recorded by Mr. Darwin are Pisum sativum and Canna 
Warscewiczi. With regard to the Garden Pea, the papilionaceous corolla is obviously 
an adaptation to insect agency by which intercrossing is secured; but the Pea in this 
country is rarely, if ever, crossed, so that cultivated varieties keep true even when grown 
together. Some of Mr. Knight’s varieties kept true for more than sixty years ; but “ their 
glory is now departed,” says Mr. Darwin*. As to the effects of intercrossing, the latter 
remarks :— 
* Owing to the varieties having been self-fertilised for many generations, and to their having been 
subjected in each generation to nearly the same conditions, I did not expect that a cross between two 
such plants would benefit the offspring ; and so it proved on trial” (l. c. p. 162). 
The ratio of the heights were as 100 : 115, * so that the crossed plants, far from beating 
the self-fertilised, were completely beaten by them." : 
Canna Warscewiczi is also highly self-fertile, the pollen being shed before the flowers 
expand. Intercrossed plants showed no advantage over the self-fertilized ; the ratio of 
the heights of the intercrossed to self-fertilized for three generations taken together was 
as 100: 101, and Mr. Darwin observes (/. c. p. 233) :— 
* We may therefore conclude that the two lots possessed equal powers of growth; aii this I believe 
to be the result of long-continued self-fertilisation, together with exposure to similar conditions in each 
generation, so that all the individuals had acquired a closely similar constitution ? +. 
In speaking of Pisum (l. c. p. 264) he says, “as the plants have been long cultivatell | 
under nearly similar conditions, we can understand why a cross between two individuals 
of the same variety does not do the least good to the offspring either in height or fer- 
tility; and he compares Pisum with his own cultivated examples of Mimulus and 
* Hero.” But the words I have italicized are an assumption which will not hold good; 
for he does not show that the soil &c. was any degree more nearly exactly the same for 
Pisum than for any other plants of which the intercrossed beat the self-fertilized. 
But the matter or question cannot rest there. One still asks why it is so. A priori 
one would be led to suppose that a plant habitually self-fertilized would, on the contrary, 
immediately be benefited by even the slightest infusion of new blood by a cross of the 
same stock, and much more by a cross from a distinct stock or different varieties. In 
the last case it is true; for by crossing varieties of Peas the ratios became at once ` 
100 : 60-70. But it is not clear why two individuals of the same variety do not benefit ` 
by a cross when they have been habitually self-fertilized. To say it is because they have ` 
* Why it is gone he does not explain ; and one would like to know whether it was from any gradually deteriorating - 
effect of self-fertilization, as he seems to imply (though the sixty years certainly afford strong presumption mE 
such being the case), or whether it was not that other marketable varieties had superseded them. 4 
t Mr. Darwin says that the opinion of some persons that this flower is invariably self-fertilized is “ am extra- - 
ordinary conclusion, for it implies that a great amount of pollen is produced for no purpose." May not the real ` 
explanation be, that while the flower was formerly adapted for insect-fertilization, as indicated by its bright ` 
perianth, abundance of pollen, &c., in the absence of the proper (native) insects it has become self-fertilizing in ` 
this country, but has retained its customary habit of developing more pollen than is actually required for self- 
fertilization? Fumaria officinalis sets but one seed, yet the amount of pollen is relatively very great. Similarly ` 
Chenopodium has retained its five fertile anthers but yields only a single seed. Many other cases might be adduced, 
in addition to those given above, to show an apparently great tit NIA: Sion Mu dert 
the ono hand, and the mambas E atida ai we es = 
