at profusion of forms among the
Hieraciums. Owing to the minute size of their florets, these plants offer
very considerable technical difficulties in the way of cross fertilisation.
By dint of great perseverance and labour, however, Mendel succeeded in
obtaining a few crosses between different forms. These hybrids were reared
and a further generation produced from them, and, no doubt somewhat to
Mendel's chagrin, every one of them proved to breed true. There was a
complete absence of that segregation of characters which he had shown to
exist in peas and beans, and had probably looked forward with some
confidence to finding in _Hieracium_. More than thirty years passed before
the matter was cleared up. To-day we know that the peculiar behaviour of
the hybrid Hieraciums is due to the fact that they normally produce seed by
a peculiar process of parthenogenesis. It is possible to take an unopened
flower and to shear off with a {134} razor all the male organs together
with the stigmata through which the pollen reaches the ovules. The flower,
nevertheless, sets perfectly good seed. But the cells from which the seeds
develop are not of the same nature as the normal ovules of a plant. They
are not gametes but retain the double structure of the maternal cells. They
are rather to be regarded as of the nature of buds which early become
detached from the parent stock to lead an independent existence, and, like
buds, they reproduce exactly the maternal characteristics. The discovery of
the true nature of this case was only rendered possible by the development
of the study of cytology, and it was not given to Mendel to live long
enough to learn why his hybrid Hieraciums all bred true.

* * * * *

{135}

CHAPTER XIII

VARIATION AND EVOLUTION

Through the facts of heredity we have reached a new conception of the
individual. Hitherto we have been accustomed to distinguish between the
members of a family of rabbits like that illustrated on Plate I. by
assigning to each an individuality, and by making use of certain external
features, such as the coat colour or the markings, as convenient outward
signs to express our idea that the individuality of these different animals
is different. Apart from this, our notions as to what constituted the
individuality in each case were at best but vague. Mendelian analysis has
placed in our hands a more precise method of estimating and expressing the
variations tha