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rts that their mutual fertilization is by so much the less easy as their differences are greater。 How far these experiments may be trusted； I know not； but the forms experimentised on； are ranked by Sagaret； who mainly founds his classification by the test of infertility； as varieties。
The following case is far more remarkable； and seems at first quite incredible； but it is the result of an astonishing number of experiments made during many years on nine species of Verbascum； by so good an observer and so hostile a witness； as G？rtner： namely； that yellow and white varieties of the same species of Verbascum when intercrossed produce less seed； than do either coloured varieties when fertilized with pollen from their own coloured flowers。 Moreover； he asserts that when yellow and white varieties of one species are crossed with yellow and white varieties of a distinct species； more seed is produced by the crosses between the same coloured flowers； than between those which are differently coloured。 Yet these varieties of Verbascum present no other difference besides the mere colour of the flower； and one variety can sometimes be raised from the seed of the other。
From observations which I have made on certain varieties of hollyhock； I am inclined to suspect that they present analogous facts。
K？lreuter； whose accuracy has been confirmed by every subsequent observer； has proved the remarkable fact； that one variety of the common tobacco is more fertile； when crossed with a widely distinct species； than are the other varieties。 He experimentised on five forms； which are commonly reputed to be varieties； and which he tested by the severest trial； namely； by reciprocal crosses； and he found their mongrel offspring perfectly fertile。 But one of these five varieties； when used either as father or mother； and crossed with the Nicotiana glutinosa； always yielded hybrids not so sterile as those which were produced from the four other varieties when crossed with N。 glutinosa。 Hence the reproductive system of this one variety must have been in some manner and in some degree modified。
From these facts； from the great difficulty of ascertaining the infertility of varieties in a state of nature； for a supposed variety if infertile in any degree would generally be ranked as species； from man selecting only external characters in the production of the most distinct domestic varieties； and from not wishing or being able to produce recondite and functional differences in the reproductive system； from these several considerations and facts； I do not think that the very general fertility of varieties can be proved to be of universal occurrence； or to form a fundamental distinction between varieties and species。 The general fertility of varieties does not seem to me sufficient to overthrow the view which I have taken with respect to the very general； but not invariable； sterility of first crosses and of hybrids； namely； that it is not a special endowment； but is incidental on slowly acquired modifications； more especially in the reproductive systems of the forms which are crossed。
Hybrids and Mongrels compared； independently of their fertility。 Independently of the question of fertility； the offspring of species when crossed and of varieties when crossed may be compared in several other respects。 G？rtner； whose strong wish was to draw a marked line of distinction between species and varieties； could find very few and； as it seems to me； quite unimportant differences between the so…called hybrid offspring of species； and the so…called mongrel offspring of varieties。 And； on the other hand； they agree most closely in very many important respects。
I shall here discuss this subject with extreme brevity。 The most important distinction is； that in the first generation mongrels are more variable than hybrids； but G？rtner admits that hybrids from species which have long been cultivated are often variable in the first generation； and I have myself seen striking instances of this fact。 G？rtner further admits that hybrids between very closely allied species are more variable than those from very distinct species； and this shows that the difference in the degree of variability graduates away。 When mongrels and the more fertile hybrids are propagated for several generations an extreme amount of variability in their offspring is notorious； but some few cases both of hybrids and mongrels long retaining uniformity of character could be given。 The variability； however； in the successive generations of mongrels is； perhaps； greater than in hybrids。
This greater variability of mongrels than of hybrids does not seem to me at all surprising。 For the parents of mongrels are varieties； and mostly domestic varieties （very few experiments having been tried on natural varieties）； and this implies in most cases that there has been recent variability； and therefore we might expect that such variability would often continue and be super…added to that arising from the mere act of crossing。 The slight degree of variability in hybrids from the first cross or in the first generation； in contrast with their extreme variability in the succeeding generations； is a curious fact and deserves attention。 For it bears on and corroborates the view which I have taken on the cause of ordinary variability； namely； that it is due to the reproductive system being eminently sensitive to any change in the conditions of life； being thus often rendered either impotent or at least incapable of its proper function of producing offspring identical with the parent…form。 Now hybrids in the first generation are descended from species （excluding those long cultivated） which have not had their reproductive systems in any way affected； and they are not variable； but hybrids themselves have their reproductive systems seriously affected； and their descendants are highly variable。
But to return to our comparison of mongrels and hybrids： G？rtner states that mongrels are more liable than hybrids to revert to either parent…form； but this； if it be true； is certainly only a difference in degree。 G？rtner further insists that when any two species； although most closely allied to each other； are crossed with a third species； the hybrids are widely different from each other； whereas if two very distinct varieties of one species are crossed with another species； the hybrids do not differ much。 But this conclusion； as far as I can make out； is founded on a single experiment； and seems directly opposed to the results of several experiments made by K？lreuter。
These alone are the unimportant differences； which G？rtner is able to point out； between hybrid and mongrel plants。 On the other hand； the resemblance in mongrels and in hybrids to their respective parents； more especially in hybrids produced from nearly related species； follows according to G？rtner the same laws。 When two species are crossed； one has sometimes a prepotent power of impressing its likeness on the hybrid； and so I believe it to be with varieties of plants。 With animals one variety certainly often has this prepotent power over another variety。 Hybrid plants produced from a reciprocal cross； generally resemble each other closely； and so it is w