258 BROWN: THE GENUS CRATAEGUS 
3. They do breed true. The progeny come with remarkable 
fidelity to the specific typical characters of the parents. 
4. They may hybridize, but I have seen no evidence of it. 
5. I have not given much attention to the conception of muta- 
tion in regard to the origin of species. It may be true. In the 
meantime I am inclined to believe in the Darwinian theory, that 
natural selection with other agencies, acting on varieties, and 
extending over a long period, has produced these Crataegus 
species, and all other species. The fact that the thorns are ex- 
ceedingly hardy, virile, and distributed over a large area, and well 
fitted to maintain the struggle for existence, no doubt has given 
them a tendency to much variation, and in the course of many 
generations these inherited “traits’’ have become fixed and 
specific, and hence, new species, after a long lapse of time, 
appeared. 
From studies, observations in the field, and what has been 
learned from special students of the genus as above set forth, we 
may arrive at certain tentative conclusions, which at least serve as 
working hypotheses. 
There is no doubt that former systematists conceived of species 
as being much more composite than they are at present regarded 
by many. The Linnaean conception prevailed. In older herbaria 
we often find two quite distinct forms, forms now regarded as dif- 
ferent species, mounted on one sheet ; this may be taken as evi- 
dence of the broader conception of species that formerly prevailed. 
Recent workers who have been studying species closely, 
especially students using the culture method, find that many sys- 
tematic species are made up of a number of distinct forms, or 
elementary species that breed true when propagated by seed. A 
notable illustration of this is Draba verna, which has been studied 
by Jordan. In many of. the descriptions of species of Crataegus 
recently offered, lines have been drawn so closely, forms separated 
by such fine distinctions that we doubtless have descriptions of 
elementary species. But this can be proved only by long culture 
experiments. 
I am inclined to think that a great many of the Crataegus 
forms we have are hybrids. European species are known to hy- 
