‘SELF-FERTILIZATION OF PLANTS. 369 
and stigmas together. Of Mr. Darwin’s experiments, Ipomæa was cultivated for the 
longest period, or ten years, This plant ought therefore to give the best results for 
showing the advantages of crossing. Omitting the tenth *, it is found that the ratio of 
the heights of the intercrossed to the self-fertilized does not steadily increase in suc- 
cessive generations, as one would à priori suppose it “ ought,” from the continued infu- 
sion of new vigour by a fresh cross every year; but it fluctuates, so that when repre- 
sented geometrically we do not obtain a regularly ascending curve but a series of 
* maxima." Hence, to ascertain the true result, it is advisable to take the mean of two 
or three years’ growth together. Dividing, therefore, the nine years into groups of three 
each, the mean ratios thus obtained of the heights of the intercrossed (given as 100) to 
those of the self-fertilized are as follows : —For 1st three generations, as 100 : 74:3; for the 
2nd three, as 100: 77:6; and for the last three, as 100 : 81:6. That is, the ratio was 
approximating unity or equality. Again, of the years in which Mr. Darwin has recorded 
the ratio of fertility, ascertained from the number of seeds per capsule; they are as 
follows :—1st year, 100 :93 ; 3rd and 4th years, 100 : 94; 5th, 100 : 106:9, and the 8th 
year as 100: 1147, which shows a steady increase in favour of self-fertilization. 
Hence, whether we look to “heights” or “fertility” as standards, there does not 
appear to be any evidence of intercrossing giving a yearly increasing, much less a 
permanent advantage. Moreover, as with Mimulus so with Ipomea, a strongly self- 
fertilizing variety sprang up, which, like Piswm and Canna, proved very impatient of 
cross-impregnation. (See remarks on Ophrys apifera, p. 978.) Mimulus luteus, 
which is self-fertilizing, gives somewhat analogous results; for, taking the ratios of | 
heights as before, the mean for the first two years is as 100 : 63:5, of the third generation 
(the mean of all the heights given, p. 66) 100 : 76, and of the fourth and fifth as 100 : 103; 
after which the “ white variety," which was so highly self-fertile, became so abundant as 
to render further experiments unnecessary, and perhaps impossible. In almost all the 
other experiments the number of generations are too few to be analyzed in this way; 
but, as a rule, they all prove that a first cross imparts some sort of stimulus ; but there is 
nothing to show that the effects would be permanent. Moreover, in all cases, some plant 
or plants of the self-fertilized beat its opponent. 
Henee I do not see how we can avoid the conclusion that the stimulus to growth 
afforded by crossing is not at all lasting, but gradually disappears, till the self-fertilized 
prove to be either equal or superior to them in the long run; so that I am led to accept 
Mr. Meehan's interpretation as follows :— 
“ Mr. Darwin’s artificial experiments seem to show, not that self-fertilization produces any injury to 
the race, but that cross-fertilization brings about a more excitable condition of growth and reproduction” f. 
The effect may be, perhaps, compared to a tonic or many other drugs, which, however 
* I omit the tenth, as the difference between the heights of the intercrossed and self-fertilized plants was so 
great as to be probably “ accidental,” so that Mr. Darwin thinks it does not give the true ratio. 
+ This is deduced from a statement on p. 39, that a self-fertilized plant produced “ the large average number of 
5:1 seeds per capsule." But as Mr. Darwin does not give that of the intercrossed, I have taken the number 4:46 
from a previous year (p. 36), which gives the ratio 100 : 114. 
t From the Pennsylvania Monthly, Philadelphia, June 1877, 
