316 
Of all parts of those variable objects which we call 
organisms, the most variable is the reproductive system ; 
and the variations may be either in the direction of in- 
creased or of diminished fertility. Having, regard, there- 
fore, to all the delicate, complex, and for the most part 
hidden conditions which determine this double kind of 
variation within the limits of the reproductive system, 
there can be no difficulty in granting that variations in 
the way of greater or less sterility must frequently occur 
both in plants and animals ina state of nature. Probably, 
indeed, if we had the means of observing this point, we 
should find that there is no one variation more common. 
But, of course, whenever it arises—whether as a result of 
changed conditions of life, or, as we say, spontaneously— 
it immediately becomes extinguished, seeing that the in- 
dividuals which it affects are less able (if able at all) to 
propagate the variation. But now, if the variation should 
be such that, while showing some degree of sterility with 
the parent form, it continues to be perfectly fertile within 
the limits of the varietal form, in this case the variation 
would neither be swamped by intercrossing, nor would it 
die out on account of sterility. On the contrary, this 
particular variation would be perpetuated with more 
certainty than any other variation, whether useful or un- 
useful. An illustration will serve to render this more clear. 
Suppose the variation in the reproductive system is 
such that the season of flowering or of pairing becomes 
either advanced or retarded. Whether this variation be, as 
we say, spontaneous, or due to any change of food, climate, 
habitat, &c., does not signify. The only point we need 
here attend to is that some individuals, living on the same 
geographical area as the rest of their species, have varied 
in their reproductive systems so that they are perfectly 
fertile zzfer se, while absolutely sterile with all other 
members of their species. By inheritance there would 
thus arise a variety living on the same geographical area 
with that form by a barrier quite as effectual as a thousand 
miles of ocean: the only difference is that the barrier, 
instead of being geographical, is physiological 
From this illustration I hope it will be obvious that 
wherever any variation in the highly variable reproduc- 
tive system occurs, tending to sterility with the parent 
form without impairing fertility with the varietal form— 
no matter whether this be due, as here supposed, to a 
slight change in the season of reproductive activity, or to 
any other cause—there the physiological barrier in question 
must interpose, with the result of dividing the species into 
two parts. And it will be further evident that when such 
a division is effected, the same conditions are furnished to 
the origination of new species as are furnished to any 
part of a species when separated from the rest by geo- 
graphical barriers or by migration. For now the two 
sections of the species, even though they be living on the 
same area, are free to develop distinct histories without 
mutual intercrossing, or, as I have phrased it, by in- 
dependent variation. 
To state this suggestion inanother form. It enables us 
to regard many, if not most, natural species as the records 
of variation in the reproductive systems of ancestors. 
When accidental variations of a non-useful kind occur in 
any of the other systems or parts of organisms, they are, 
as a rule, immediately extinguished by intercrossing. But 
whenever they happen to arise in the reproductive system 
in the way here suggested, they must inevitably tend to 
be preserved as new natural varieties, or incipient species. 
At first the difference would only be in respect of the re- 
productive system ; but eventually, on account of inde- 
pendent variation, other differences would supervene, and 
the new variety would take rank as a true species. 
The principle thus briefly sketched in some respects 
resembles, and in other respects differs from, the principle 
of natural selection. or survival of the fittest, as I will 
show later on. For the sake of convenience, therefore, 
NATURE 
| circumstances which materially affect our verdict. 
as its parent form, and yet prevented from intercrossing | 
[August 5, 1886 
and in order to preserve analogies with already existing 
terms, I will call this principle Physiologica] Selection, or 
Segregation of the Fit. 
Before proceeding to state the evidence of the particular 
kind of variation on which this principle depends, let it 
be noted that we are not concerned either with its 
causes or its degrees. Not with its causes, because in 
this respect the theory of physiological selection is 
in just the same position as that of natural selection: 
it is enough for both that the needful variations are pro- 
vided, without its being incumbent on either to explain the 
causes which underlie them. Neither are we concerned 
with the degrees of sterility which the variation in question 
may in any particular case supply. For whether the 
degree of sterility with the parent form be originally 
great or small, the result of it will in the long run be the 
same: the only difference will be that in the latter case a 
greater number of generations would be required in order 
to separate the varietal from the parent form. 
(To be continued.) 
TROPICAL FRUITS? 
"ae present Colonial and Indian Exhibition has deve- 
loped interest in tropical fruits to an extent not pre- 
viously known in England; and whatever may be the indi- 
vidual merits of many of the fruits displayed in the colonial 
market attached to the Exhibition, no one can deny that 
they afford proof of numerous undeveloped resources of 
our colonial possessions in a direction hitherto very much 
undervalued or entirely overlooked. Sir Joseph Hooker, 
in one of his journals, has remarked that “ most tropical 
fruits are edible, but few are worth eating.” But, after 
all, the merits of many fruits like those of certain wines 
are only properly appreciated under a concourse of local 
In 
the tropics the desire for refreshment and for something 
cooling and piquant is met by a fruit which, at the 
moment,completely answers the purpose. Transferred to a 
cooler climate, the same fruit may be entirely out of place, 
and indeed may be condemned as valueless. As a case 
in point, the water of a young cocoa-nut, when clear and 
fresh, taken from the fruit after a long ride in the sun, is 
most refreshing and wholesome. The same thing tried 
| in the climate of England, and with fruit imported from 
the tropics, would be nauseous and wholly unpalatable. 
Similar remarks would apply to the fruit of the mango- 
steen, the durian, and many others where it is necessary 
that the fruit should be eaten when just ripe, and where 
a long journey affects the quality and impairs the delicacy 
—hboth being of an evanescent character. 
Again, it is necessary to bear in mind how to use 
certain tropical fruits in order to appreciate them to the 
best advantage. Owing to the loose manner in which 
tropical fruits are termed, apples, plums, pears, peaches, 
&c., when they are neither botanically nor intrinsically 
anything of the sort, there has arisen considerable 
confusion respecting them. Again, many tropical fruits 
are suitable only for salads or curries, and should not 
appear at the dessert table at all. Others are better 
when preserved or cooked, and they are then both whole- 
some and well adapted to the wants of the country. 
There is no good pear (Pyrus communis, L.), as known in 
England, grown in the tropics, yet we have the name 
applied to the Alligator or Avocado pear (Persea gra- 
tissima, Gart), the anchovy pear (Grias caulifiora, L.), 
the prickly pear (Opuntia ficus-indica, Webb), and the 
wooden pear of Australia (Xy/omelum pyrtforme, Knight). 
Again, the English apple, although grown in the hills in 
the tropics, is practically of little value, but the name is 
| as loosely applied as in the case of the pear, and hence 
fruits as widely apart as the poles in their botanical classi- 
t-Lecture given in the Conference Hall, Colonial Exhibition. Revised by 
the Author. 
