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Periodicity in the Organic World : 
| May, 
VI. PERIODICITY IN THE ORGANIC WORLD : 
A PROLEGOMENON. 
S OW the Periodic Law has been applied to the classifi- 
cation of minerals, it seems natural to raise the 
question whether any harmony, not indeed identical, 
but at least approximately analogous, can be traced in the 
animal and vegetable kingdoms ? We do not, cf course, refer 
to that well-known periodicity which may be found morpho- 
logically in the recurrence of certain forms and relations in 
the individual being, nor yet to the alternate or cyclical repeti- 
tion of phenomena in its development and life. In these 
directions much has been already learnt, and more remains 
as yet unlearnt. But we have in view taxonomy, — the 
classification of organic forms. We venture to ask whether, 
on viewing, e. g., the animal kingdom as a whole, we can 
trace in it anything which shall remind us, however remotely, 
of the Newlands-Mendelejeff classification of the chemical 
elements. Do we find any phenomenon repeating itself at 
definite intervals ? Two a priori objections will here at once 
suggest themselves. It may be said that the very attempt 
to search out such correspondences in the organic world is 
hostile to evolutionism. To this charge we demur. We 
consider that the periodicity so far traced among the chemical 
elements should rank not against but in favour of their having 
been evolved on definite laws from some common pristine 
matter. In like manner, if any cyclical repetition should be 
traced in the animal and vegetable kingdoms, such results 
will by no means militate against evolution, even if it may 
tend to relegate “ Natural Selection ” to a less important 
position than that which it now occupies. 
The second objection is of much graver character. Among 
the elements we have a certain important variable : the 
atomic weight. The task of the discoverers of the Periodic 
Law has been to exhibit the properties of the elements as 
periodic functions of such variable. But in plants and ani- 
mals we know of nothing comparable to the atomic weights 
of the elements. Hence if we proceed at all in this inquiry 
it can only be by an inverse method : by inquiring what pro- 
perties vary in any particular group, and recur varying in a 
corresponding manner in other groups. After tracing out 
such variations, it will then be necessary to inquire if there 
