lO/o The American Naturalist. [December, 
one mode of motion would generate its successor in obedience to 
the molecular structural record first laid down in the ovum and 
spermatozooid, and then combined and recomposcd on the union 
of the two in the oospore, or fertilized ovum. 
Were all cells identical in characters, everyone would retain the 
structural record, or memory of its past physical history, as do the 
unicellular organisms. Evolution has however so modified most 
of the structural units of the organic body that none but the nervous 
and reproductive cells retain this record, in greater or less perfection. 
And the nervous cells have been specialized as the recipients of 
new impressions, and the excitors of definite corresponding move- 
ments in the cells of the remainder of the organism. The somatic 
cells retain only the record or memory of their special function. On 
the other hand, the reproductive cells, which most nearly resemble 
the independent unicellular organisms, retain first the impressions 
received during their primitive unicellular ancestral condition ; 
and second, those which they have acquired through the organ- 
ism of which they have been and are only a part. And the prin- 
cipal medium through which they can receive such impression 
is that system of cells called the nervous system, which has been 
specialized through use and natural selection to receive impres- 
sions from without, and to transmit them to such parts of the 
organism as are capable of receiving them. And the only other 
cells which can retain and record the entire record are the re- 
productive cells. 
This is the logical result of the considerations which have oc- 
cupied the preceding pages, and is the carrying out of the 
Bathmism theory of heredity, of which I have given hitherto 
only the bare outline. 
• Since Darwin, successive contributions have been made to the 
theory of heredity in its relation to evolution. In 1878 and 187 1 
the present writer advanced the dynamic hypothesis, but made no 
attempt to explain the mode of conveyance of dynamic impres- 
sions and modifications to the germ-cells. The theory of " peri- 
genesis " proposed by Haeckel in 1873 is of the same character, 
and is deficient in the same way. The modified pangenesis theory 
of Brooks, published in 1883, attempts to supply the defect found 
