Dear Sir

Your question respecting the relative value of species in times past & present, can only, I fear, be answered hypothetically—¹

1. The proportion of species to genera was only 7 to 1 in the Silurian Period; it rose to 14 in the Jurassic, & 16 in the Eocene Tertiary. At the present day there are 40 sp. to each genus of shells, on the average, according to my estimate (p. 417)² Now as shells are very simple contrivances presenting few points of comparison, (for in this inquiry we must set aside colour) the difficulty of determining species must increase very rapidly with the number in each genus. There are 3 or 400 sp. of Conus & 4 or 500 Pleurotomas—so that any new species possessing the essential characters of either genus will probably run very close to some sorts already known— The number of possible variations of these types must be well-nigh exhausted.

It does not follow from this that the species are bad—ie varieties of a common stock.

On the other hand, fossil shells present difficulties, on account of their condition, which usually more than make up for their comparative fewness.

When suites of fossils can be obtained in good condition—such as the Bayeux fossils & the U. Greensand sp. of Mans, which are like recent shells—they present just the same degrees of specific variation as their living representatives

In Deshayes Fossils of the Paris Basin about half the species are good, according to my notion of species.³ In his catalogues of recent shells (Veneridæ, Cycladidæ, Tellinidæ) perhaps one fourth are good for something.⁴

The accurate Dr Louis Pfeiffer seems to have grown weary, or disgusted with the task of discriminating Helicidæ after describing 3000 species, & tossed in another odd 900 species sans discretion—perhaps to please Mr Cuming.⁵

Mr Gray says only 110 of the reported species of shells are really distinct;⁶ & you know the rest of the joke. Henceforth the definition of “Species” will be—“a little group of Genera”.— “Genus; the smallest term in Zoology; something less than a variety”.

2. Your question may be answered by appealing to Geographical Distribution. D^r Hooker says “the species in isolated islands are generally well-defined; this is in part the consequence of a law that genera in islands bear a large proportion to the species” (Antarctic Botany.)⁷ I believe this to be true, & that the Island faunas are older (generally) than those of Continents. Still it cannot be denied that the Helicidæ of Madeira exhibit a high degree of specific variation— Even in the more ancient S^t. Helena have we not Bulimus auris-vulpinus & B. Darwini? And if you appeal to the Galapagos Ids I must almost admit the converse of the above proposition!

3. Lastly we may compare the relative value of species in Genera of different degrees of antiquity. The worst genus of all—Clausilia is at least of Eocene date—its 280 species are restricted to the Lusitanian Province, save a few which range through the Germanic to China.⁸

The Terebratulæ of the Oolites are not more variable than the Terebratellæ of the present seas— Mr Buckman’s attempt to demolish some of them was founded on a comparison of young & undeveloped specimens—⁹ Some of these have internal characters with which he was unacquainted. In every case the period of maximum development of a genus may be taken as the time at which the diagnosis of its species is most difficult—& the entire fauna of each period presents the same aspect of a combination of groups, some commencing, others culminating, the rest declining. It is no use pursuing this further, as I cannot give you a matter-of-fact answer. The truth is that the species of every group, country & time, cannot be distinguished empirically, but require judgment & experience—& this will be no new truth to you—

Pray when you can, let me know what points in my book I ought to re-consider!

Yrs sincerely | S. P. Woodward