56 BBITISH ASSOCIATION FOE THE ADVANCEMENT OE SCIENCE. 



disunited by the catastrophes which supervened ; in the other, we may regard the 

 peculiar Flora now existing as merely the wreck, as it were, of one which once over- 

 spread a large tract of land, of which all but the little patch on which it is now 

 found had been since submerged. Upon this subject our opinions may in some 

 measure be swayed by the nature of the conclusion we arrive at with respect to the 

 length of time during which seeds are capable of mantaining their vitality; for if 

 after remaining for an indefinite period in the earth they were capable of germina- 

 ting, it would doubtless be easier to understand the revival, under favorable cir- 

 cumstances, of plants which had existed before the severance of a tract of land from 

 the continent in which they are indigenous. An inquiry has accordingly been car- 

 ried on for the last fifteen years under the auspices of, and with the aid of funds 

 supplied by, this Association, the results of which, it is but fair to say, by no means 

 corroborate the reports that had been from time to time given us with respect to the 

 extreme longevity of certain plants, exemplified, as it was said, in the case of the 

 mummy-wheat and other somewhat dubious instances ; inasmuch as they tend to show, 

 that none of the seeds which were tested, although they were placed under the most 

 favorable artificial conditions that could be devised, vegetated after a period of for- 

 ty-nine years; that only twenty out of 288 species did so after twenty years; 

 whilst by far the larger number had lost their germinating power in the course of 

 ten. These results, indeed, being merely negative, ought not to outweigh such pos- 

 itive statements on (he contrary side as come before us recommended by respectable 

 authority, such, for instance, as that respecting a Nelurnbium seed, which germina- 

 ted after having been preserved in Sir Hans Sloane's Herbarium for 150 years ; still, 

 however, they throw suspicion as to the existence in seeds of that capacity of pre- 

 serving their vitality almost indefinitely, which alone would warrant us in calling to 

 our aid this principle in explaining the wide geographical range which certain spe- 

 cies of plants affect. 



************* 



Amongst the many services rendered to the Natural Sciences by Dr. Hooker, in 

 conjunction with his fellow traveller, Dr. Thomson, one of the greatest I conceive 

 to be, that they have not only protested against that undue multiplication of species, 

 which had taken place by exalting minute points of difference into grounds of rad- 

 ical and primary distinction, but that they have also practically illustrated their 

 views with respect to the natural families which have been described by them in the 

 volume alluded to. They have thus contributed materially to remove another 

 difficulty w T hich stood in the way of the adoption of the theory of specific centres, 

 — I mean the replacement of forms of vegetation in adjoining countries by others, 

 not identical, but only as it should seem allied ; for it follows from the principles 

 laid down by these authors, that such apparently distinct species may after all 

 have been only varieties, produced by the operation of external causes acting upon 

 the same species during long periods of time. 



But if this be allowed, what limits, it may be asked, are we to assign to the 

 changes which a plant is capable of undergoing, — and in what way can we oppose 

 the principle of the transmutation of species, which has of late excited so much 

 attention, and the admission of which is considered to involve such startling con- 

 sequences? I must refer you to the writings of modern physiologists for a full 

 discussion of this question. All that I shall venture to remark is, that had not 

 Nature herself assigned certain boundaries to the changes which plants are capable of 

 undergoing, there would seem no reason why any species at all should be restricted 



