REPRODUCTION BY SEED 165 
cross-pollination that they were inclined to minimize the 
importance and extent of self-fertilization, which was 
regarded as not merely undesirable, but, as a general rule, 
positively harmful to the race. This is an exaggeration, 
for in most cases self-fertilization can certainly take place 
for many generations without impairing in any way the 
vigour of the stock. At the same time, the importance of 
cross-fertilization in maintaining a good average race is 
unquestioned, and Robert Knight, in 1837, was no doubt 
right in the main when he said that self-fertilization was 
not possible for a perpetuity of generations without the 
intervention of cross-fertilization. 
The truth of the matter is that self-fertilization has no 
more harmful effect on future generations than vegetative 
reproduction ; probably less. Vitiated habits and objec- 
tionable characters acquired by a parent are transmitted 
by vegetative multiplication, but not by seed, even when 
self-fertilized, unless they have become so pronounced as 
to impair the nutrition and check the development of 
their possessor. The same is true of in-breeding in 
animals. If the parents are sound, the progeny is sound. 
The danger arises when unsoundness comes in. By in- 
breeding this is intensified ; by cross-breeding it is modified 
or eliminated. Again, self-pollination is economical, a 
large number of seeds being fertilized with the minimum 
expenditure of pollen. Moreover, the certainty of pollina- 
tion is greater than in cross-pollinated plants, and generally 
the output of seed in these plants is very great indeed. 
This accounts for the wide distribution of autogamous 
species, and the rapidity with which they conquer new 
ground. Many of the commonest and most widespread 
weeds are self-pollinated—e.g., groundsel (Senecio vul- 
garis), chickweed (Stellaria media), shepherd’s-purse (Cap- 
sella Bursa-pastoris). These all have small inconspicuous 
flowers which rarely open. Many other weeds are only 
occasionally cross-pollinated, and even in ordinary insect- 
pollinated flowers self-pollination very often steps in 
when cross-pollination fails. The garden-pea, in spite of 
its attractions, is invariably self-pollinated. In some 
plants seeds are formed in flower-buds which never open. 
These closed or cleistogamous buds (Gr. cleistos, closed) 
are found in many species of Viola, wood-sorrel (Oxalis), 
and stitchwort. These plants are visited by few insects 
