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always is； speaking broadly； to reproduce the form of the parents。 The
proverb has it that the thistle does not bring forth grapes； so； among
ourselves； there is always a likeness； more or less marked and
distinct； between children and their parents。 That is a matter of
familiar and ordinary observation。 We notice the same thing occurring
in the cases of the domestic animalsdogs； for instance； and their
offspring。 In all these cases of propagation and perpetuation； there
seems to be a tendency in the offspring to take the characters of the
parental organisms。 To that tendency a special name is given it is
called 'Atavism'； it expresses this tendency to revert to the ancestral
type； and comes from the Latin word 'atavus'； ancestor。
Well； this 'Atavism' which I shall speak of； is； as I said before； one
of the most marked and striking tendencies of organic beings； but； side
by side with this hereditary tendency there is an equally distinct and
remarkable tendency to variation。 The tendency to reproduce the
original stock has； as it were； its limits； and side by side with it
there is a tendency to vary in certain directions； as if there were two
opposing powers working upon the organic being； one tending to take it
in a straight line； and the other tending to make it diverge from that
straight line； first to one side and then to the other。
So that you see these two tendencies need not precisely contradict one
another； as the ultimate result may not always be very remote from what
would have been the case if the line had been quite straight。
This tendency to variation is less marked in that mode of propagation
which takes place asexually； it is in that mode that the minor
characters of animal and vegetable structures are most completely
preserved。 Still； it will happen sometimes； that the gardener； when he
has planted a cutting of some favourite plant； will find； contrary to
his expectation； that the slip grows up a little different from the
primitive stockthat it produces flowers of a different colour or
make； or some deviation in one way or another。 This is what is called
the 'sporting' of plants。
In animals the phenomena of asexual propagation are so obscure； that at
present we cannot be said to know much about them； but if we turn to
that mode of perpetuation which results from the sexual process； then
we find variation a perfectly constant occurrence； to a certain extent；
and； indeed； I think that a certain amount of variation from the
primitive stock is the necessary result of the method of sexual
propagation itself； for； inasmuch as the thing propagated proceeds from
two organisms of different sexes and different makes and temperaments；
and as the offspring is to be either of one sex or the other； it is
quite clear that it cannot be an exact diagonal of the two； or it would
be of no sex at all； it cannot be an exact intermediate form between
that of each of its parentsit must deviate to one side or the other。
You do not find that the male follows the precise type of the male
parent； nor does the female always inherit the precise characteristics
of the mother；there is always a proportion of the female character in
the male offspring； and of the male character in the female offspring。
That must be quite plain to all of you who have looked at all
attentively on your own children or those of your neighbours； you will
have noticed how very often it may happen that the son shall exhibit
the maternal type of character； or the daughter possess the
characteristics of the father's family。 There are all sorts of
intermixtures and intermediate conditions between the two； where
complexion； or beauty； or fifty other different peculiarities belonging
to either side of the house； are reproduced in other members of the
same family。 Indeed； it is sometimes to be remarked in this kind of
variation； that the variety belongs； strictly speaking； to neither of
the immediate parents； you will see a child in a family who is not like
either its father or its mother； but some old person who knew its
grandfather or grandmother； or； it may be； an uncle； or； perhaps； even
a more distant relative； will see a great similarity between the child
and one of these。 In this way it constantly happens that the
characteristic of some previous member of the family comes out and is
reproduced and recognised in the most unexpected manner。
But apart from that matter of general experience； there are some cases
which put that curious mixture in a very clear light。 You are aware
that the offspring of the Ass and the Horse； or rather of the he…Ass
and the Mare； is what is called a Mule； and； on the other hand； the
offspring of the Stallion and the she…Ass is what is called a 'Hinny'。
I never saw one myself； but they have been very carefully studied。 Now；
the curious thing is this； that although you have the same elements in
the experiment in each case； the offspring is entirely different in
character； according as the male influence comes from the Ass or the
Horse。 Where the Ass is the male； as in the case of the Mule； you find
that the head is like that of the Ass； that the ears are long； the tail
is tufted at the end； the feet are small； and the voice is an
unmistakable bray； these are all points of similarity to the Ass； but；
on the other hand； the barrel of the body and the cut of the neck are
much more like those of the Mare。 Then； if you look at the Hinny；the
result of the union of the Stallion and the she…Ass； then you find it is
the Horse that has the predominance； that the head is more like that of
the Horse； the ears are shorter； the legs coarser； and the type is
altogether altered； while the voice； instead of being a bray； is the
ordinary neigh of the Horse。 Here； you see； is a most curious thing：
you take exactly the same elements； Ass and Horse； but you combine the
sexes in a different manner； and the result is modified accordingly。
You have in this case； however； a result which is not general and
universalthere is usually an important preponderance； but not always
on the same side。
Here； then； is one intelligible； and； perhaps； necessary cause of
variation： the fact； that there are two sexes sharing in the production
of the offspring； and that the share taken by each is different and
variable； not only for each combination； but also for different members
of the same family。
Secondly； there is a variation； to a certain extentthough； in all
probability； the influence of this cause has been very much
exaggeratedbut there is no doubt that variation is produced； to a
certain extent； by what are commonly known as external conditions；such
as temperature； food； warmth； and moisture。 In the long run； every
variation depends； in some sense； upon external conditions； seeing that
everything has a cause of its own。 I use the term 〃external
conditions〃 now in the sense in which it is ordinarily employed： certain
it is； that external conditions have a definite effect。 You may take a
plant which has single flowers； and by dealing with the soil； and
nourishment； and so on； you may by…and…by convert single flowers into
double flowers； and make thorns shoot out into branches。 You may
thicken or make various modifications in the shape of the fruit。 In
animals； too； you may produce analogous changes in this way； as in the
case of that deep bronze colour which persons rarely lose after having
passed any length of time in tropical countries。 You may also alter
the development of the muscles very much； by dint of training； all the
world knows that exercise has a great effect in this way； we always
expect to find the arm of a blacksmith hard and wiry； and possessing a
large development of the brachial muscles。 No doubt training； which is
one of the forms of external conditions； converts what are originally
only instructions； teachings； into habits； or； in other words； into
organizations； to a great extent； but this second cause of variation
cannot be considered to be by any means a large one。 The third cause
that I have to mention； however； is a very extensive one。 It is one
that； for want of a better name； has been called 〃spontaneous
variation；〃 which means that when we do not know anything about the
cause of phenomena； we call it spontaneous。 In the orderly chain of
causes and effects in this world； there are very few things of which it
can be said with truth that they are spontaneous。 Certainly not in
these physical matters；in these there is nothing of the
kind；everything depends on previous conditions。 But when we cannot
trace the cause of phenomena； we call them spontaneous。
Of these variations； multitudinous as they are； but little is known with
perfect accuracy。 I will mention to you some two or three cases；
because they are very remarkable in themselves； and also because I
shall want to use them afterwards。 Reaumur； a famous French naturalist；
a great many years ago； in an essay which he wrote upon the art of
hatching chickens；which was indeed a very curious essay；had
occasion to speak of variations and monstrosities。 One very remarkable
case had come under his notice of a variation in the form of a human
mem