Thus the whole series does not run:

_a, b, d, e, f, g, h, i, k,_ &c.,

but

_a, b, c, d_, A, B, C, D, alpha, beta, gamma, delta, &c.;

so that it is said to express a _periodic law_ of recurrent
similarities. Or the relation may be expressed in another way. In each
section of the series, the atomic weight is greater than in the
preceding section, so that if _w_ is the atomic weight of any element
in the first segment, _w+x_ will represent the atomic weight of any
element in the next, and _w+x+y_ the atomic weight of any element in
the next, and so on. Therefore the sections may be represented as
parallel series, the corresponding terms of which have analogous
properties; each successive series starting with a body the atomic
weight of which is greater than that of any in the preceding series,
in the following fashion:

_d_ D delta
_c_ C gamma
_b_ B beta
_a_ A alpha
- ----- ---------
_w_ _w + x_ _w + x + y_

[Sidenote: The possibility of a primary form of matter.]

This is a conception with, which biologists are very familiar, animal