416 The Biology of Flowers 
different varieties. “It is really wonderful what an effect pollen 
from a distinct seedling plant, which has been exposed to different 
conditions of life, has on the offspring in comparison with pollen from 
the same flower or from a distinct individual, but which has been long 
subjected to the same conditions. The subject bears on the very 
principle of life, which seems almost to require changes in the 
conditions'.” 
The fertility—measured by the number or weight of the seeds 
produced by an equal number of plants—noticed under different 
conditions of fertilisation may be quoted in illustration. 
On crossing | On crossing 
with a fresh | plants of the | ¢ eter 
stock same stock spun 
Mimulus luteus 
(first and ninth generation) 100 4 3 
Eschscholzia californica 
(second generation) 100 45 40 
Dianthus caryophyllus 
(third and fourth generation) 100 45 33 
Petunia violacea 100 54 46 
Crossing under very similar conditions shows, therefore, that the 
difference between the sexual cells is smaller and thus the result of 
crossing is only slightly superior to that given by self-fertilisation. Is, 
then, the favourable result of crossing with a foreign stock to be 
attributed to the fact that this belongs to another systematic entity or 
to the fact that the plants, though belonging to the same entity were 
exposed to different conditions? This is a point on which further 
researches must be taken into account, especially since the analysis 
of the systematic entities has been much more thorough than 
formerly, We know that most of Linnaeus’s species are compound 
species, frequently consisting of a very large number of smaller or 
elementary species formerly included under the comprehensive term 
varieties. Hybridisation has in most cases affected our garden and 
cultivated plants so that they do not represent pure species but a 
mixture of species. 
But this consideration has no essential bearing on Darwin's point 
of view, according to which the nature of the sexual cells is in- 
1 More Letters, Vol. 11. p. 406. 
? In the case of garden plants, as Darwin to a large extent claimed, it is not easy to 
say whether two individuals really belong to the same variety, as they are usually of hybrid 
origin. In some instances (Petunia, Iberis) the fresh stock employed by Darwin possessed 
flowers differing in colour from those of the plant crossed with it. 
