6 The Law of Evolution : is it General? [January, 
But if we look at a table of the elements, and consider 
their properties, we can scarcely fail to be reminded of the 
features presented by a group of organic forms. In such a 
group— say in a large genus of inserts — we find a few spe- 
cies common, numerous, and well-marked. We then see a 
number of others comparatively scarce, or indeed very rare. 
Among these rare species we find several which are difficult 
to distinguish from each other, and we often trace sub-groups 
inhabiting the same district, and having an especially strong 
family likeness. We find species standing widely apart from 
each other, and between which some conneCting-link seems 
either to have disappeared or to have escaped notice, and 
others again so closely similar that the discovery of any 
further allied species would be felt as an inconvenience. 
Again, if we try to arrange animal forms in any regular 
series, according as they depart gradually from some given 
type, we proceed for a while very satisfactorily ; but before 
long we come to a species which seems equally well entitled 
to two or even more positions. 
If a digression is here permissible we may ask whether 
something like periodicity does not obtain in the organic 
world, and whether an indistinct recognition of this principle 
—overlaid indeed with assumptions and errors in abundance 
— was not the grain of truth hidden in the “ quinarian ” or 
representative system of Macleay, Swainson, and their 
allies ? Naturalists who consider that species, like indivi- 
duals, are evolved not at haphazard, but in certain given 
directions, will perhaps see here a clue to an advance in 
taxonomy.* 
But to return : these very features which we have just 
traced* — say in the Coleopterous genus Aphodius — may be in 
the main recognised in our table of chemical elements. 
Here, again, we find a few common species and many rare 
ones, the latter generally clinging together, being often diffi- 
cult to discriminate, and inhabiting some special geological 
formation or some mineral species, just as a zoological sub- 
genus is found in some island like Macassar or Madagascar. 
We do not, indeed, find among the elements any two whose 
distinct nature remains for a length of time doubtful. But 
we come upon difficulties of classification very similar to 
those encountered in the organic world, aberrant and ano- 
malous members which have not been fully reduced to order 
even by the periodic classification. 
We have pointed out the abruptness of chemical combi- 
* We shall probably, by opportunity, resume this question. 
