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TRANSACTIONS Of SECTION D, 763 
The conjugation of the ovum and spermatozoon in the higher animals, and 
the corresponding process in the higher plants, are special cases of this conjuga- 
tion, in which special conjugating individuals are produced, the ordinary indi- 
viduals being physically incapable of the process. The phenomenon of sex, with 
all its associated complications, which is so characteristic of the higher animals 
and plants, is merely a device to ensure the coming together of the two gametes. 
In the lower animals it is possible for the ordinary organism to conjugate ; con- 
sequently the phenomenon does not form the precursor of developmental change, 
and is in no way associated with reproduction. Indeed, in such cases it is often 
the opposite of reproduction, inasmuch as it brings about a reduction in the 
number of individuals, two separate individuals fusing to form one. 
Acquired Characters. 
We now come to the consideration of the second kind of variations—namely, 
those which owe their origin to the direct action of external agencies upon the 
particular organism which shows the variation; or, as Darwin puts it, to the 
definite action of external conditions These are the variations which I have 
called acquired variations or acquired characters. This is not a good name for 
them, but at the present moment, when I am about to submit them to a critical 
examination, I do not know of any other which could be suitably applied. Later 
on, when I sum up the various effects of the direct action of external agencies 
upon the organism, I may be able to use a more suitable term. 
The main peculiarities of acquired variations are two in number: (a) they 
make their appearance as soon as the organism is submitted to the changed con- 
ditions ; (4) speaking generally they are more or less the same in all the individuals 
of the species acted upon. As examples of this kind of variations, I may mention 
the effect of the sun upon the skin of the white man; the Porto Santo rabbit, an 
individual of which recovered the proper colour of its fur in four years under the 
English climate; the change of Artemia salina to Artemia milhausenii; the 
increase in size of muscles as the result of exercise; and the development of any 
special facility in the central nervous system. Among plants, variations of this 
kind are very easily acquired, by altering the soil and climate to which the 
individuals are submitted. So common are they, that it is quite possible that a 
large number of species are really based upon characters of this kind; characters 
which are produced solely by the external conditions, and which frequently dis- 
appear when the old conditions are reverted to. 
With regard to these variations, we want to ask the following question: Do 
they ever last after the producing cause of them is removed, and are they trans- 
mitted in reproduction? In a great number of cases they either cease when the 
cause which has produced them is removed, or if they last the life of the individual 
they are not transmitted in reproduction. But is this always the case? That is 
the important question we now have to consider. 
But before doing so let us inquire what acquired characters really are. The 
so-called adults of all animals have, as part of their birthright, a certain plasticity 
in their capacity of reactiny to external influences; they all have a certain power 
of acquiring bodily and mental characters under the influence of appropriate 
stimuli. This power varies in degree and in quality in different species. In 
plants, for instance, it is mainly displayed in habit of growth, form of foliage, &c.; 
In man in mental acquirements, and so on. But however it is displayed, it is 
this property of organisms which permits of the acquisition of those modifications 
of structure which have been so widely discussed as acquired characters. Now 
this power, when closely considered, is in reality only a portion of that capacity 
for development which all organisms possess, and with which they become 
endowed at the act of conjugation. A newly formed zygote possesses a certain 
number of hidden properties which are not able to manifest themselves unless it 
is submitted to certain external stimuli. It is these stimuli which constitute the 
external conditions of existence, and the properties of the organism which are 
1 Darwin, Variation, ed, 2, vol. i. p. 119. 
