Biological History: Strickland on Zoological Systematics

By | 2012/02/28

In looking at the Wallace biogeography flap, I came across an interesting passage in Wallace’s 1855 Sarawak paper:

We shall thus find ourselves obliged to reject all those systems of classification which arrange species or groups in circles, as well as those which fix a definite number for the divisions of each group. The latter class have been very generally rejected by naturalists, as contrary to nature, notwithstanding the ability with which they have been advocated; but the circular system of affinities seems to have obtained a deeper hold, many eminent naturalists having to some extent adopted it. We have, however, never been able to find a case in which the circle has been closed by a direct and close affinity. In most cases a palpable analogy has been substituted, in others the affinity is very obscure or altogether doubtful. The complicated branching of the lines of affinities in extensive groups must also afford great [[p. 188]] facilities for giving a show of probability to any such purely artificial arrangements. Their death-blow was given by the admirable paper of the lamented Mr. Strickland, published in the ‘Annals of Natural History,’ in which he so clearly showed the true synthetical method of discovering the Natural System.

Hmmm, OK. What are the odds that one could manage to lay hands on a paper published about 172 years ago? Google search for “Annals of Natural History Strickland” placed an Internet Archive link high in the ranking. That page offers the text of the paper in several different formats.

So what does Mr. Strickland say about systematic work?

The postulate with which I commence the inquiry is, to let
it be granted that there are such things as species, distinct in their characters and permanent in their duration. This being admitted, we define the natural system to be the arrangement of species according to the degree of resemblance in their essential characters. In other words, the natural system is that arrangement in which the distance from each species to every other is in exact proportion to the degree in which the essential characters of the respective species agree. Hence it follows that the whole difficulty of discovering the natural system consists in forming a right estimate of these degrees of resemblance. For the degree in which one species resembles another must not be estimated merely by the conspicuousness or numerical amount of the points of agreement, but also by the physiological importance of these characters to the existence of the species. On this point no certain rules have yet been laid down ; for though naturalists in general admit, for instance, that the nervous system is superior in importance to the circulatory, and the latter superior to the digestive system, yet this subject is still in a very indeterminate state, and until our knowledge of physiology is much further advanced, disputes will always arise respecting the true position of certain species in the natural classification. Such differences of opinion, however, will continually diminish as our knowledge increases, and they are even now very few in comparison with the numerous facts in classification on which all naturalists are agreed. Much may be effected by education and habit, which impart to the naturalist a peculiar faculty (termed by Linnaeus a ” latent instinct 5 ‘) for appreciating the relative importance of physiological characters to the satisfaction of himself and others, even in cases where he is unable to explain the principles which determine his decision.

Strickland devotes the bulk of his paper, though, to a thorough trashing approaches to systematics that proposes some ordering principle from without. Linear arrangements, numerological arrangements, and circular arrangements all come in for deconstruction and dismissal.

The best part I see, though, is Strickland’s argument for why variety is and must be the aspect of nature that zoologists simply have to accede to.

2. It follows from the irregularity of external nature, as seen on the surface of the earth, that the groups of organized beings must be irregular also, both in their magnitudes and in their affinities. In proof of this it must be granted that the final cause of the creation of every animal and plant is the discharge of a certain definite function in nature, and not the mere occupation of a certain post in the classification : in short, that the design of creation was to form not a cabinet of curiosities, but a living world. Few, I trust, would hesitate to admit this proposition. If, then, the different modifications of structure which constitute the characters of groups were given solely with reference to the external circumstances in which the creature is destined to live, it follows that the irregularities of the external world must be impressed upon the groups of animals and of plants which inhabit it. The supply of organic beings is exactly proportioned to the demand ; and Nature does not, for the sake of producing a regular classification, go out of her way to create beings where they are not wanted, or where they could not subsist. Thus, for instance, the warm climate and varied soil of the tropics admits of the growth of a vast variety of flowers and fruits. The group of Humming-birds which feed on the former, and of Parrots which feed on the latter, are accordingly found to be developed in a vast variety of generic and specific forms ; while the family of Gulls which seek their food in the monotonous and thinly inhabited regions of the north, are few in species and still fewer in genera. Again, the variety of plants in the tropics admits the existence of a great variety of insects, and the family of woodpeckers is proportionately numerous; while the Oxpecker {Buphaga) % which seems to form a group fully equivalent in value to the Woodpeckers, is limited to but one or two species, because its food is confined to a few species of insects which only infest the backs of oxen.

It follows, then, that the groups of organized beings will be great or small, and the series of affinities will be broken or
continuous, solely as the variations of external circumstances
admit of their existence, and not according to any rule of
classification. If, indeed, we were to imagine a world laid
out with the regularity of a Chinese garden, in which a certain number of islands agreeing in size, shape, soil, and form of surface, were placed at exactly equal distances on both sides of the equator, we might then conceive the possibility of a perfect symmetry in the groups of beings which inhabit them ; but without some such supposition, I do not see how a class of animals or plants can be symmetrical in themselves, and yet be expressly adapted for conditions of existence which are eminently irregular.

This statement of Strickland’s appears to express the concept of niche that Joseph Grinnell would be credited with some sixty-seven years later in 1917. There is the persistent difficulty in looking at almost all Victorian-era naturalist writings pre-Origin-of-Species that everything has to be couched in terms of some sort of creationary framework. But the citation of Strickland in Wallace’s 1855 paper does show a nice progression in the history of ideas, where a concept of dependence of a species on a set of environmental conditions leads to the concept of biogeography relating species not just to particular constraints, but also to particulars of place and time in relation to parent and daughter species.

The particular proposal of Strickland’s, to evaluate characters weighted in some way by importance to the species in order to assess affinities to other species, markedly differs from what is considered current today. The cladistic approach developed in the 1960s explicitly gets rid of “weighting” schemes and the notion that a few well-understood characters are better for assessing affinity than many characters simply noted as present or absent. So Strickland’s actual proposal of what the true method of discovering the natural system would be hasn’t held up, but several of his reasons for rejecting prior methods still carry weight, and his expression of this appears to have contributed to the development of biogeography as a topic.

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