1885.] 
a Prolegomenon. 
283 
is anything of which these changes can be said to be periodic 
functions. The question is vastly complicated by the absence 
of the numencal clement which plays so important a part in 
the Iifc.ess world. Among the chemical elements we can 
ex hmit certain important relations, such as specific gravities, 
malleabilities, dualities, atomic volumes, manners and 
ranges of combining. with oxygen, chlorine, or sulphur, &c., 
as 1 educed to numerical standards. 
. ^ ve }y important reas on in favour of making the attempt 
is involved in the principle of continuity. We see Evolution 
extending from suns to microbia. In an analogous manner, 
ravmg seen 1 enodicity established in the inorganic elements 
and their natural compounds, the attempt to recognise this 
principle, also in the living compounds of matter is at least 
not illegitimate. 
It. will at once, strike all persons whose memory or whose 
reading extends into the former half of the present century, 
that an attempt,— perhaps rather a group of attempts,— was 
made to exhibit the organic world arranged on the prin- 
ciples of recurrence, or, in other words, of periodicity. This 
though undoubtedly made, and that by men of great learning-, 
was ti ammelled by a variety of baseless assumptions, and still 
more by its open and avowed contempt for all considerations 
save those drawn from outwardly visible structure. Hence 
whatever evidence or guidance might have been reaped from 
p lysiological or from embryological considerations, and in 
P a ^t from morphology also, was necessarily wanting. 
rh.us it may, we think, fairly be submitted that the 
principle of organic periodicity per se is not absolutely and 
foi ever discredited by the admitted failures of MacLeay 
owainson, and their colleagues. 
As a preliminary it will, at least, be useful to examine 
whether the chemical elements of which living bodies are 
composed hold any well-marked position in the Periodic 
classification ? 
The reply is in the affirmative. If we look at the table 
given, e.g., in Mr. Muir’s “Treatise on the Principles of 
Chemistry (p. 225), we see that among the twelve series 
there given these elements are present in the 1st, 2nd, 3rd, 
4th, 5th, and 7th only. In the 7th one only of these ele- 
ments (iodine) is present, and in the 5th three : copper, zinc, 
and bromine. Now of these four elements, copper, though* 
widely disseminated among plants, and met with in a few 
animals, occurs in small quantities only, and its especial 
funaion is in most cases still a matter of doubt. Zinc is 
much rarer, whilst iodine and bromine are chiefly confined 
