216 
Heredity. 
" Division of labour would produce a differentiation of the single cells 
in such a colony : thus, certain cells would be set apart for obtaining' food 
and for locomotion, while certain other cells would be exclusively reproduc- 
tive. In this way, colonies consisting of somatic and of reproductive cells 
must have arisen, and among these, for the tirst time, death appeared. For 
in each case the somatic cells must have perished after a certain time, while 
the reproductive cells alone retained the immortality inherited from the 
Protozoa " [Amoebae, Infusoria, &c.]. 
One more extract must suffice to indicate, in this brief manner, 
the direction of Weisniann's argument. Premising that by " germ- 
plasm" he means "the reproductive subtance," he says : — 
" "We have an obvious means by which the inheritance of all transmitted 
peculiarities takes place in the continuity of the substance of the germ-cells or 
germ-plasm. If, as I believe, the substance of the germ-cells — the germ- 
pi ism — has remained in perpetual continuity from the tirst origin of li!e, and 
if the germ-plasm and the substance of the body — the somato-plasm — have 
always occupied different spheres; and if changes in the latter only arise 
when they have been preceded by corresponding changes in the former, then 
we can, up to a certain point, understand the principle of heredity; or, at 
any rate, we can conceive that the human mind may at some time be capable 
of understanding it." 
And it is concluded, "that heredity depends upon the continuity 
of the molecular substance of the germ from generation to genera- 
tion." 
There exists at the present time no more warmly-controverted 
doctrine than that concerning the transmissibility of acquired 
characters. Weismann's definition of an acquired character is not 
very satisfactory, but here it is : — 
" An organism cannot acquire am thing unless it already possesses the 
predisposition to acquire it; acquired characters are, therefore, no more than 
local, or sometimes general, variations which arise under the stimulus pro- 
vided by certain external influences." 
He is quite opposed to the idea that acquired characters are 
transmissible, and on this point finds himself at issue with some of 
the most prominent of our English biologists. "His argument is : — 
" If — as it seems to me — the facts of the case compel us to reject the 
assumption of the transmission of acquired characters, there only remains one 
principle by which we can explain the transformation of species — the direct 
alteration of the germ-plasm, however we may imagine that such alterations 
have been produced and combined to form useful modifications of the 
body." 
Many of the illustrations are of great interest. The main root of 
a plant, it is argued, has not acquired the power of growing perpen- 
dicularly downwards under the stimulus of gravity because this 
force lias acted upon it for numberless generations, but because such 
a direction for such a part was the most useful to the plant. Can 
it be supposed, it is asked, that the brown scales which form the 
characteristic protective covering of winter buds have been produced 
by the direct action of the cold ? The fact that Hoffmann succeeded 
