SEPTEMBER 21, 1899] 
form of reproduction. Among bacteria alone, so far as I know, 
has the phenomenon of conjugation never been observed. 
To sum up, we have seen that the phenomenon of conjuga- 
tion is very widely distributed. Excluding the bacteria, there 
is reason to believe that it forms a part of the vital phenomena 
of all organisms. Its essential features are a mixture and fusion 
of the protoplasm of two different organisms, accompanied by a 
fusion of their nuclei. It results in the formation of a new 
individuality, which differs from the individualities of both the 
conjugating organisms. This difference manifests itself in 
differences in habit, constitution, form, and structure, such 
differences constituting what we have called genetic variations. 
The conjugation of the ovum and spermatozoon in the higher 
animals, and the corresponding process in the higher plants, are 
special cases of this conjugation, in which special conjugating 
individuals are produced, the ordinary individuals being physi- 
cally 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; consequently 
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 
T 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 ; (2) they make their appearance as soon as the organism 
is submitted to the changed conditions ; (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 upen 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 ;1 the change of Artemia salina to Artemia milhausenit ; 
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 disappear 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 transmitted in reproduction? Ina great 
number of cases they either cease when the cause which has pro- 
duced 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 reacting 
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 acquire- 
ments, and so on. But however it is displayed, it is this pro- 
perty of organisms which permits of the acquisition of those 
modifications of structure which have been so widely discussed 
as acguiyed characters. Now this power, when closely con- 
1 Darwin, ‘‘ Variation,” ed. 2, vol. i, p. 119. 
NO. 1560, VOL. 60] 
NATURE 
505 
sidered, is in reality only a portion of that capacity for develop- 
ment which all organisms possess, and with which they become 
endowed at the act of conjugation. A newly formed zygote pos- 
sesses 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 condi- 
tions of existence, and the properties of the organism which are 
only displayed under their influence are what we call acquired 
characters. They are acquired in response to the external 
stimuli. 
It would appear, then, that every feature which successively 
appears in an organism in the march from the uninucleated 
zygote to death is an acquired character. At first the stimuli 
which are necessary are quite simple, being little more than ap- 
propriate heat and moisture ; ]ater on they become more com- 
plicated, until finally, when the developmental period is over 
and the mature life begins, the necessary conditions attain their 
greatest complexity, and their fulfilment constitutes what we 
call in the higher animals education. Education is nothing 
more than the response of the nearly mature organism to ex- 
ternal stimuli, the penultimate response of the zygote to ex- 
ternal stimuli, the ultimate being those of senile decay, which 
end in natural death, Acquired properties, it will be seen, are 
really stages in the developmental history. They differ in the 
complexity of the stimulus required to bring them out. For 
instance, the segmentation of the egg requires little more than 
heat and moisture, the walking of the chick the stimulus of 
light and sound and gravity, the evolutions of an acrobat the 
same in greater complexity, and, lastly, the action of a statesman 
requires the stimulation of almost every sense in the greatest 
complexity. Moreover, not only are there differences in the 
complexity of the stimulus required, but also in the rapidity 
with which the organism reacts to it. The chick undergoes its 
whole embryonic development in three weeks, a man in nine 
months; the chick develops its walking mechanism in a few 
minutes, while a man requires twelve months or more to effect 
the same end. Chickens are much cleverer than human beings 
in this respect. There is the same kind of difference between 
them that there is between the power of learning displayed by 
a Macaulay and that displayed by a stupid child. 
An instinct is nothing more than an internal mechanism which 
is developed with great rapidity in response to an appropriate 
stimulus. It is difficult for us to understand instincts, because 
with us almost all developmental processes are extremely slow 
and gradual, This particularly applies to the development of 
those nervous mechanisms, the working of which we call 
reason, 
Within certain limits the external conditions may vary without 
harming the organism, but such variations are generally accom- 
panied by variations in the form in which the properties of the 
zygote are displayed. If the variations of the conditions are too 
great, their action upon the organism is injurious, and results in 
abortions or death. And in no case can the external conditions 
call out properties with which the zygote was not endowed at the 
act of conjugation. 
It would thus appear that acquired characters are merely 
phases of development ; they are the manifestations of the pro- 
perties of the zygote, andare called forth only under appropriate 
stimulation ; moreover, they are capable of varying within certain 
limits, according to the nature of the stimulus, and it is to 
these variations that the term acquired character has been 
ordinarily applied, 2h 
A genetic character, on the other hand, is the possibility of 
acquiring a certain feature under the influence of a certain 
stimulus ; it is not the feature itself—that is an acquired character 
—but it is the possibility of producing the feature. Now as the 
possibility of producing the feature can only be proved to exist 
by actually producing it, the term genetic character is frequently 
applied to the feature itself, which is, as we have seen, an 
acquired character. In consequence of this fact, that we can 
only determine genetic characters by examining acquired char- 
acters, a certain amount of confusion may easily arise, and has 
indeed often arisen, in dealing with this subject. This can be 
avoided by remembering that in describing genetic characters 
account must always be taken of the conditions. For example, 
the white fur of the Arctic hare is an acquired character, 
acquired in response to a certain stimulus ; while the power of 
so responding to the particular stimulus when applied at the cor- 
rect time is a genetic character. Thus a genetic character is a 
character which depends upon the nature of the organism, 
