132 Darwin on the Effects of Gross- and Self- Fertilization. 



" Each of these lists contains by a mere accident the same number 

 of genera, viz., forty-nine. The genera in the first list include 

 sixty-live species, and these in the second sixty species; the Orchi- 

 dese in both being excluded. If the genera in this latter order, 

 as well as in the Asclepiadese and Apocynacese, had been included, 

 the number of species which are sterile if insects are excluded 

 would have been greatly increased ; but the lists are confined to 

 species which were actually experimented on. The results can be 

 couriered a* only approximately accurate, tor fertility is so vari- 

 able a character, that each species ought to have been tried many 

 times. The above number of species, namely, 125, is as nothing 

 to the hosts of living plants; but the mere fact of more than half 

 of them being sterile within the specified degree, when insects 

 are excluded, is a striking one; for whenever pollen has to be 

 carried from the anthers der to ensure full fer- 



tilitv. there is at least a good chance of cross-fertilization. I do 

 not, however, believe that if all known plants were tried in the 

 sane manner, half would be found to be sterile within the specified 

 limits; for many flowers were select ; which pre- 



sented some remarkable structure; and such Mowers often require 

 insect-aid." (p. 370.) 



It is won m and T. pratense (the 



common white and red clovers) have a place in the first list ; T. 

 an-euxii am! T. procnuth- as in the second. Darwin refers to Mr. 

 Miner's statement that " in the United States, hive-bees never 

 suck the red clover," and says it is the same in England, ex- 

 cept from the outside through holes bitten by humble-bees ; 

 yet that H. Miiller has seen them visiting this plant in Ger- 

 many, for the sake both of pollen and nectar, which latter they 

 obtained by breaking apart the petals. Darwin has not qual- 

 ified his statement, long ago made, of the complete sterility of 

 red clover protected from insects ; but Mr. Meehan asserts 

 that protected plants are fertile in this country, without, how- 

 ever, giving details or the rate of fertility. In T. arvense, " the 

 excessively small flowers are incessantly visite~d by hive and 

 humble-bees; when insects were excluded the flower-heads 

 seem to produce as many and as fine seeds as the exposed 



As to cross-fertilization, " the most important of all the means 

 by which pollen is carried from the anthers to the stigma of the 

 same flower, or from flower to flower, are insects, belonging to 

 the orders of Hymenoptera, Lepidoptera, and Diptera ; and in 

 some parts of the world, birds." In a note the author cites all 

 the cases known to hi .-flowers. These are 



chiefly humming-birds. "In North America thev are said to 

 frequent the flowers of Impatiens " (for which Gould", Trochilidae, 

 is referred to as ith .rity. i d a reference is given to the Garden- 

 ers' Chronicle, which we find relates to something else in South 



