480 SECULAR CHANGES IN PLANTS. 



in a multitude of instances, accomplished by the use of seeds, and this 

 precisely in the instance in which we should be led to expect it. Of 

 our kitchen-garden plants, the carrot, the beet, the turnip, the cabbage, the 

 pea, etc., we propagate the expected kind without any uncertainty by the 

 use of seeds, never supposing that they will run back to the wild stock, 

 or give origin to plants different to those from which they were derived. 

 The care of man, exerted for many years upon these vegetables, has, then, 

 impressed upon them a change very far from ephemeral in its nature, and 

 enabled them to pass from the condition of mere varieties into that of 

 actual sub-species. 



Acknowledging, therefore, the influence which physical agents exert on 

 Necessity of the growth and development of plants, and admitting that 

 time forchan^ f avorm g circumstances will bring on a modification of form, 

 ing plants. especially if applied long enough, and that man himself, by 

 his arts of culture, can, without difficulty, establish similar variations, we 

 might be led to expect that more profound changes in external circum- 

 stances, if steadily applied through extended periods of time, would give 

 origin to more striking results. A variation in the constitution of the air, 

 in the brilliancy of light, in the mean temperature, moisture, or chemical 

 constitution of the soil, if kept up for thousands of years, or permanent- 

 ly established, could not fail to exert a prodigious effect upon the whole 

 vegetable world. If, for example, the brilliancy of the sun in the slow 

 lapse of centuries should gradually decline, or the mean temperature of 

 the surface of the earth should descend, or enormous quantities of car- 

 bonic acid be permanently removed from the air and replaced by equiva- 

 lent volumes of oxygen gas ; if carbonate of lime, to an extent sufficient 

 for the formation of geological strata, were removed from the waters, in 

 which it could no longer be held in solution because of the withdrawal 

 of carbonio acid from the atmosphere, it must follow, as a matter of inev- 

 itable necessity, that the whole vegetable world would feel the change. 

 Plants that at one time existed could exist no more ; others, by gradu- 

 ally accommodating themselves to the slow revolution, would exhibit 

 here the development of one part, there the development of another, and 

 some, which perhaps maintain themselves with difficulty under the old 

 state of things, would now begin to develop themselves in a more luxu- 

 riant way. 



The changes here spoken of hypothetically have, however, actually oc- 

 Secuiar changes cur *ed in the history of the earth. We can not shut our 

 occurring to the eyes to the corresponding!; march which vegetation has made, 



earth and occa- J . .* ,? i i A - vo_ ^.i. 



commencing in the earliest geological times with the stem- 



turns in plants. i ess cryptogamia, followed by those provided with stems 

 and leaves, the gymnospores, such as the conifers and cycadeas, next 

 making their appearance, after these, monocotyledons, and at last the 



