242 Robertson: Theortes of Evolution. 
were transmitted was unknown because the nature of fertilisation 
was completely misunderstood. For a long time after the dis- 
covery of the male fertilising element in the seventeenth 
century it was believed that the spermatozoon itself developed 
into the offspring, and that the only part played by the female 
was in nursing and protecting it. We now know that the 
essential thing in fertilisation is the union of two nuclei, the egg 
nucleus of the female and the sperm nucleus of the male. 
In investigating the laws of heredity experimentally it is 
obvious that results will be more easily obtained if we choose 
for the parents individuals which differ considerably ; that is to 
say, we shall get more information by observing the manner in 
which a character peculiar to one parent appears in the offspring 
than by investigating a character common to both. For 
instance, if we fertilise the ovules of a yellow-seeded pea with 
pollen from a green-seeded pea, and find that the next gener- 
ation bears only yellow seeds, we may infer that the seedling's 
have inherited their seed-colour from the female parent, whereas 
if we had used yellow-seeded peas for both parents we should 
not know whether the yellowness in the next generation was 
due to the influence of the male parent, or the female parent, or 
both. The way in which the presence of the greatest number 
of differentiating characters can be secured is by working with 
hybrids, that is to say, crossing individuals belonging — to 
different races. 
The most remarkable work that has ever been done on plant 
hybrids is that of Mendel, and is of such extreme importance 
that I must treat of his classical paper at some length. 
Gregor Mendel (1822-1884) was the son of a peasant in Austrian 
Silesia, and entered an Augustinian Monastery, of which he 
eventually became Abbot. All his experiments were made in 
the convent garden, and his great paper on pea hybrids was 
published in an obscure local journal in 1865. It was quite 
overlooked by biologists for more than thirty years, but since it 
has been unearthed we might almost say that it has revolutionised 
our ideas as to the nature of living things. Mendel began his 
work by looking about for some species which should be as 
convenient as possible to grow and to experiment with, and 
the different races of which should be quite fertile when crossed 
together, and should produce fertile hybrids. He finally hit 
upon Pisum sativum, the ordinary eating pea. This species 
has the great advantage of always fertilising itself when left 
alone, so that there is no risk of accidental crossing: Twenty- 
Naturalist, — 
