DE SAEOETA. TEETIAEY ELOEA OE PEOVENCE. 
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is of opinion that like external conditions (convenances exterieures 
physiques on clhnateriques) determine like structural modifications, 
and in this way he explains these remarkable and familiar analogies. 
It would not be easy to show why like external conditions should not 
determine like modifications, though the given conditions must be 
regarded, at the same time, as the agents of natural selection—being, 
indeed, both the determining cause and selecting agent of variation 
•—but we think it can scarcely be said that experience has, as yet, 
materially confirmed the importance of external agents as the pri¬ 
mary determining causes of variation. Their influence as selecting 
agents there can be no question of, though the consideration of their 
role as such is greatly complicated by important disturbing elements. 
These questions are too profound to enter upon here. The view 
which M. de Saporta holds upon them must, we think, undergo some 
modification, on a careful consideration of the recent writings of 
Messrs. Darwin and Hooker, which no one occupied in such specu¬ 
lative inquiries ought to ignore, and which may lead him further, to 
regard from a different point of view the old hypothesis which he 
favours, that species and types have, like the individual, their irrevo¬ 
cable term. He writes, “ Mais, en dehors de ces causes destructives 
“ (submergence, lowering of temperature, &c.) soit rapides, soit 
“ lentes, il en existe une autre inherente a la nature meme des choses 
“ creees, incessamment active, quoique d’une maniere latente et par 
“ des precedes inconnus: e’est celle qui se rattache au mode de de- 
“ veloppement des types organiques. En effet, les types vegetaux 
“ semblent doues d’une vie qui leur est propre” 
A study of Tertiary vegetation, in respect to the succession and 
mutual relations of the various types, leads M. de Saporta to recognise 
as a “ loi tres-importante dans Tetude des vegetaux fossiles,” that 
each group ‘ £ se comporte a travers le temps comme il se comporte 
“ a tr avers Vespace .” He goes on to affirm “ qu’il suffit de preciser 
“ le caractere, les allures et la physionomie d’un groupe, la nature 
“ des combinaisons auxquelles il donne lieu, pour counaitre en meme 
“ temps quel a du etre son role dans le passe.” In support of this 
notion, he adduces, among genera at present numerous in species— 
Pims, Q,uercus , Ficus, and Qinnamomum, —as formerly exhibiting 
like “ fecondite,” and a group now rich in form—Proteaceae—as 
having been anciently in like manner varied: genera which are re¬ 
stricted now, as Elm, Alder, and Hornbeam, formerly presented 
“inevitables repetitions des memes formes.” We have no question 
at all that Tertiary data abundantly suffice to substantiate the facts 
M. de Saporta here states. We see in them, however, but further 
confirmation of the soundness of the observations of Mr. Darwin in 
the chapter on ‘ The Geological Succession of Organic Beings, 7 in 
his c Origin of Species. 7 Indeed the theory of this distinguished 
Naturalist finds in M. de Saporta’s “ loi tres-importante” but the 
feeble expression of a subsidiary corollary. Deferring to the physi¬ 
ognomy of Tertiary vegetation, De Saporta states that, in the lower 
