PART 1. ] HISTORICAL INTRODUCTION. - B. 
in the next year, Darwin produced his Origin of Species, and in it 
emphasised Knight’s law as a general law of nature, placing it on 
broader and surer foundations and uniting it intimately with his 
theory of natural selection. This theory showed for the first time 
the full value of Sprengel’s work, and caused his book, which had 
been forgotten for seventy years, to play a prominent part in the 
investigation of the prime causes which determine the forms of 
flowers. 
As a foundation for the hypothetic natural law that “no organic 
being fertilises itself for a perpetuity of generations, but that a 
cross with another individual is occasionally—perhaps at very long 
intervals—indispensable,’” 1 Darwin showed that in all higher and 
the great majority of lower animals the sexes are separate, and 
that most hermaphrodite forms pair regularly ; that, in the experi- 
ence of breeders of animals and cultivators of plants, breeding 
in-and-in diminishes the strength and the productiveness of the 
offspring, while crossing with another breed, or with another stock 
of the same breed, increases both ; that, according to the above- 
mentioned experiments of several botanists, the application of 
pollen to the pistil of the same flower is less efficient than pollen 
from another individual; that in very many plants the situation of 
the reproductive organs, exposed to the weather and often liable to 
injury, may be most simply explained if we admit the necessity of 
occasional crossing; that, according to his own experiments on 
Papilionacee, the exclusion of insect-visits in many cases diminishes 
or arrests productiveness; that, as Sprengel had shown in many 
cases and Darwin had confirmed, self-fertilisation is prevented in 
many flowers by the relative positions of the reproductive organs 
or by their ripening at different times; finally, that in no living 
organism do the structure or situation of the reproductive organs 
prevent occasional crossing with another individual of the same 
species. These statements, taken separately, were neither decisive 
nor free from objection, but collectively they lent a high degree of 
probability to Darwin's hypothesis; and so, from its close connec- 
tion with the question of the origin of species and the fundamental 
importance that it therefore had for all botanical research, botanists 
could not help at once taking part for or against it, according to 
whether they were impelled by the general weight of evidence or 
deterred by the gaps in the chain. 
The opposers justly maintained that though in the animal 
kingdom the possibility of occasional pairing might be admitted in 
1 Origin of Species, chap. iv. ‘On the Interciossing of Individuals.”’ 
