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PRINCIPLES OF BIOLOGY. 
Variation. 
No organism is exactly like its parent. Variation is 
co-extensive with Heredity, and the evidence which proves 
Heredity in its smaller manifestations is the evidence which 
proves variation, for it is only when there occur variations 
that anything beyond the transmission of structural peculi¬ 
arities can be proved. 
The transmission of variations is itself variable. An 
individual trait in one parent may be counteracted in the 
offspring, or may appear in an equal, or in a less, or in a 
higher degree. An instance of this is cited from Dr. Strothers 
of a family in which the transmission of digital increase was 
traced through four generations varying in position and degree. 
Though unlikeness among progenitors is one antecedent of 
variation, it is by no means the only one, for successive 
offspring of the same parent are never exactly alike. This is 
accounted for by the functional variation of the parents, and 
is shown by the fact that twins are more nearly alike than 
children born in succession. But why are not seeds 
out of one pod and animals born at one birth exactly 
alike ? There is another cause for variation yet to seek. 
In any series of dependent changes a small initial 
difference often works a marked difference in the result— 
instance the great likeness that exists between all babies a 
few weeks old. And again, no two parts of any aggregate 
can be similarly conditioned with respect to incident forces. 
Hence, no two ova, no two ovules, no two spermatozoa, no 
two pollen-cells can be identical, and the reproductive centres 
must begin to differentiate from the very outset. 
The inferences from the power that organisms display of 
reproducing lost parts is, that the units of which an organism 
is built have an innate tendency to arrange themselves into 
the organism, but as reasons have been given for believing 
that the reproductive cells are not highly specialized, and it 
was actually seen in one of the organisms that the units of 
each undifferentiated cell were capable of arranging them¬ 
selves into the form of species, we are driven to the assump¬ 
tion, as Spencer says:— 
(1.) That sperm cells and germ cells are essentially nothing 
more than vehicles in which are contained small groups of 
the physiological units in a fit state for obeying their pro¬ 
clivity towards the structural arrangement of the species they 
belong to. 
(2.) That the likeness of any organism to either parent 
is conveyed by the special tendencies of the units derived 
from the parents. 
