If we look at
species as having been specially created, and at varieties as having
been produced by secondary laws, this similarity would be an
astonishing fact. But it harmonises perfectly with the view that there
is no essential distinction between species and varieties.
SUMMARY OF CHAPTER.
First crosses between forms sufficiently distinct to be ranked as
species, and their hybrids, are very generally, but not universally,
sterile. The sterility is of all degrees, and is often so slight that
the two most careful experimentalists who have ever lived, have come
to diametrically opposite conclusions in ranking forms by this test.
The sterility is innately variable in individuals of the same species,
and is eminently susceptible of favourable and unfavourable
conditions. The degree of sterility does not strictly follow
systematic affinity, but is governed by several curious and complex
laws. It is generally different, and sometimes widely different, in
reciprocal crosses between the same two species. It is not always
equal in degree in a first cross and in the hybrid produced from this
cross.
In the same manner as in grafting trees, the capacity of one species
or variety to take on another, is incidental on generally unknown
differences in their vegetative systems, so in crossing, the greater
or less facility of one species to unite with another, is incidental
on unknown differences in their reproductive systems.
Pages:
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337