Comparative Physiology. 335 



dependent on stimuli. He distinguishes between mechanical 

 stimuli, as pressure, &c., which exhaust, and vital, which re- 

 novate. " Nutriment is not merely a stimulus of the organic 

 body ; it is itself susceptible of life ; it is a stimulus which vivi- 

 fies, and can itself receive vitality. The constant reanimation of 

 the tissues by the general vital stimuli ordinarily renders them 

 capable of a proportionate exercise of their functions ; but, if 

 their action is increased and accelerated, subsequent rest is 

 necessary to restore as much power for ncAV action as has been 

 thus consumed. Rest alone, however, induces weakness; the 

 power of an organ is increased by exercise, not carried too far, 

 and alternating with rest." The affinity of the tissues for the 

 vital stimuli seems to be greater where the developement is less 

 complete, or in the young state. That the power of vitality 

 is increased by its exercise, I have often before noticed ; it is 

 from this cause that a lazy slow-growing plant is converted, by 

 cutting in the shoots, into a more vigorous groAvth ; the activity 

 given by the start made after cutting continues. Sets from 

 vigorous-grown potatoes, and cuttings and seeds from vigorous - 

 grown plants, always thrive best. The bad consequences, how- 

 ever, of too much and too long continued excitement in plants, 

 without a due proportion of rest, have been often pointed out, 

 and have been most conspicuous in the forcing of the vine. Much 

 of the bad consequences attributed to excitement are probably 

 also due to a want of balance in the effects of stimuli. Were 

 it possible to follow up the great expansion by heat with a 

 corresponding degree of light, the weakness produced by the 

 former might not be so apparent ; and great part of what is 

 ascribed to over-stimulus may be rather due to a deficiency of 

 other stimuli, to a want of light, as well as too much heat. 



That the embryo is susceptible of changes at the time of form- 

 ation, of which it is not susceptible afterwards, is clearly to be 

 seen in the many hybrids produced ; and those hybrids being 

 more frequent among plants, shows that the lower grades of 

 beings are more susceptible of these changes. Whether any or 

 what portion of the change is due to stimuli, we are, however, 

 perhaps unable to decide. Whether the germ is produced by 

 the male or female, or the joint influence of both, in seeds, can 

 hardly be said to be decided. There may also be much owing 

 to certain kinds and states of food being required for the de- 

 velopement of parts, without which they cannot be produced. 

 Of this kind seems the transformation from a leaf bud to a 

 flower ; the greater degree of elaboration of the sap seeming all 

 that is requisite to produce from a bud, the germ of a branch, 

 so apj^arently different an organ as a flower. That it is a 

 real transformation seems evident from the many changes ob- 

 served, from petal to leaf, from stamen to petal, and from a 



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