equally have to be verified inferentially. If we still hold to the idea of Linnæus, and of Agassiz, that existing species were created independently and essentially all at once at the beginning of the present era, we could not better the propositions of Linnæus and of Jussieu. If, on the other hand, the time has come in which we may accept, with De Candolle, their successive origination, at the commencement of the present era or before, and even by derivation from other forms, then the "in principio" of Linnæus will refer to that time, whenever it was, and his proposition be as sound and wise as ever.

In his "Géographie Botanique" (ii., 1068–1077) De Candolle discusses this subject at length, and in the same interest. Remarking that of the two great facts of species, viz., likeness among the individuals, and genealogical connection, zoölogists have generally preferred the latter,' while botanists have been divided in opinion, he pronounces for the former as the essential thing, in the following argumentative statement :

Quant à moi, j'ai été conduit, dans ma definition de l'espèce, à mettre décidément la ressemblance au-dessus de caractères de succession. Ce n'est pas seulement à cause des circonstances propres au règne végétal, dont je m'occupe exclusivement; ce n'est pas non plus afin de sortir ma définition des théories et de la rendre le plus possible utile aux naturalistes descripteurs et nomenclateurs, c'est aussi par un motif philosophique. En toute chose il faut aller au fond des questions, quand on le peut. Or, pourquoi la reproduction est-elle possible, habituelle, féconde indefiniment, entre des êtres organisés que nous dirons de la

'Particularly citing Flourens: "La ressemblance n'est qu'une condition secondaire; la condition essentielle est la descendance: ce n'est pas la ressemblance, c'est la succession des individus, qui fait l'espèce."

même espèce? Parce qu'ils se ressemblent et uniquement à cause de cela. Lorsque deux espèces ne peuvent, ou, s'il s'agit d'animaux supérieurs, ne peuvent et ne veulent se croiser, c'est qu'elles sont très differentes. Si l'on obtient des croisements, c'est que les individus sont analogues; si ces croisements donnent des produits féconds, c'est que les individus étaient plus analogues; si ces produits eux-mêmes sont féconds, c'est que la ressemblance était plus grande; s'ils sont fécond habituellement et indéfiniment, c'est que la ressemblance intérieure et extérieure était très grande. Ainsi le degré de ressemblance est le fond; la reproduction en est seulement la manifestation et la mesure, et il est logique de placer la cause au-dessus de l'effet."

We are not yet convinced. We still hold that genealogical connection, rather than mutual resemblance, is the fundamental thing-first on the ground of fact, and then from the philosophy of the case. Practically, no botanist can say what amount of dissimilarity is compatible with unity of species; in wild plants it is sometimes very great, in cultivated races often enormous. De Candolle himself informs us that the different variations which the same oak-tree exhibits are significant indications of a disposition to set up separate varieties, which becoming hereditary may constitute a race; he evidently looks upon the extreme forms, say of Quercus Robur, as having thus originated; and on this ground, inferred from transitional forms, and not from their mutual resemblance, he includes them in that species. This will be more apparent should the discovery of transitions, which he leads us to expect, hereafter cause the four provisional species which attend Q. Robur to be merged in that species. It may rightly be replied that this conclusion would be arrived at from the likeness step

by step in the series of forms; but the cause of the likeness here is obvious. And this brings in our "motif philosophique."

Not to insist that the likeness is after all the variable, not the constant, element-to learn which is the essential thing, resemblance among individuals or their genetic connection-we have only to ask which can be the cause of the other.

In hermaphrodite plants (the normal case), and even as the question is ingeniously put by De Candolle in the above extract, the former surely cannot be the cause of the latter, though it may, in case of crossing, offer occasion. But, on the ground of the most fundamental of all things in the constitution of plants and animals the fact incapable of further analysis, that individuals reproduce their like, that characteristics are inheritable-the likeness is a direct natural consequence of the genetic succession; "and it is logical to place the cause above the effect."

We are equally disposed to combat a proposition of De Candolle's about genera, elaborately argued in the "Géographie Botanique," and incidentally reaffirmed in his present article, viz., that genera are more natural than species, and more correctly distinguished by people in general, as is shown by vernacular names. But we have no space left in which to present some evidence to the contrary.

V.

SEQUOIA AND ITS HISTORY; THE RELATIONS OF NORTH AMERICAN TO NORTHEAST ASIAN AND TO TERTIARY VEGETATION.

(A PRESIDENTIAL ADDRESS TO THE AMERICAN ASSOCIATION FOR THE ADVANCEMENT OF SCIENce, at Dubuque, August, 1872.)

THE session being now happily inaugurated, your presiding officer of the last year has only one duty to perform before he surrenders the chair to his successor. If allowed to borrow a simile from the language of my own profession, I might liken the President of this Association to a biennial plant. He flourishes for the year in which he comes into existence, and performs his appropriate functions as presiding officer. When the second year comes round, he is expected to blossom out in an address and disappear. Each president, as he retires, is naturally expected to contribute something from his own investigations or his own line of study, usually to discuss some particular scientific topic.

Now, although I have cultivated the field of North American botany, with some assiduity, for more than forty years, have reviewed our vegetable hosts, and assigned to no small number of them their names and their place in the ranks, yet, so far as our own wide country is concerned, I have been to a great extent a

closet botanist. Until this summer I had not seen the Mississippi, nor set foot upon a prairie.

To gratify a natural interest, and to gain some title for addressing a body of practical naturalists and explorers, I have made a pilgrimage across the continent. I have sought and viewed in their native haunts many a plant and flower which for me had long bloomed unseen, or only in the hortus siccus. I have been able to see for myself what species and what forms constitute the main features of the vegetation of each successive region, and record—as the vegetation unerringly does-the permanent characteristics of its climate.

Passing on from the eastern district, marked by its equably distributed rainfall, and therefore naturally forest-clad, I have seen the trees diminish in number, give place to wide prairies, restrict their growth to the borders of streams, and then disappear from the boundless drier plains; have seen grassy plains change into a brown and sere desert-desert in the common sense, but hardly anywhere botanically so-have seen a fair growth of coniferous trees adorning the more favored slopes of a mountain-range high enough to compel summer showers; have traversed that broad and bare elevated region shut off on both sides by high mountains from the moisture supplied by either ocean, and longitudinally intersected by sierras which seemingly remain as naked as they were born; and have reached at length the westward slopes of that high mountain-barrier which, refreshed by the Pacific, bears the noble forests of the Sierra Nevada and the Coast Ranges, and among them trees which are the