10 INTRODUCTION. H^- 



becomes exhausted. The perennial phmt puts forth phyton after phyton, but the 

 seed is necessary to its perpetuation. Numerous lower animals are reproduced to a 

 vast extent by segmentation or allied processes, but ultimately a recurrence to sexual 

 admixture becomes necessary for the preservation of the species.^ Sexual admix- 

 ture, limited to a few families of a species, soon ends in their extinction. Finally, 

 the complex living being, from birth to death, produces an immensity of living or- 

 ganisms, the organic cells ; but the egg and seed are necessary to insure the species 

 against extinction. 



Living beings did not exist upon earth prior to their indispensable conditions of 

 action, but wherever these have been brought into operation concomitantly, the 

 former originated ; and for such an immensity of time and vastness in quantity, have 

 they existed, that most of the superficial rocks of the earth's crust are composed of 

 their remains. 



The stratum of life has been always subjected to the destructive agency of earth- 

 quakes, volcanoes, and torrents ; but it is wonderful how soon, under the play of the 

 life-conditions, the new surfoce again teems with living beings. Here and there, 

 upon the wide area of the earth, an igneous rock peeps out as if to observe the 

 monopoly of life, but even this, in the progress of time, has its steep sides hidden 

 by lichens and its summit enveloped in verdure. 



Of the life, present everywhere with its indispensable conditions, and coeval m 

 its origin with them, what was the immediate cause ? It could not have existed 

 upon earth prior to its essential conditions; and is it, therefore, the result of these? 



There appear to be but trifling steps from the oscillating particle of inorganic 

 matter, to a Bacterium ; from this to a Vibrio, thence to a Monas, and so gradually 

 up to the highest orders of life ! The most ancient rocks containing remains of 



^ Instances in favor of this view are numerous; among others, I have met with a striking example in the 

 case of a worm, to which I have given the name of Stylana fossularis (Proc. Acad. Nat. Sci. V. 287). 

 This worm is found abundantly in ditches in the neighborhood of Philadelphia, during warm weather, and is 

 constantly observed to be undergoing division. Individuals, a third of an inch long, are usually found to 

 consist of two divisions, and occasionally of three, in various stages of progress towards separation. The 

 divisions are composed of about twenty-two annulations, each possessed of a pair of fasciculi of five podal 

 spines and two bristles. The head consists of a large lobe, with a long digit-like appendage, and presents 

 an eye upon each side of a large mouth. The latter opens into a capacious pharynx, which afterwards con- 

 tracts into a cylindrical oesophagus, continuous with a well-developed intestine, but within the animal no 

 trace of a generative apparatus can be perceived. 



In the course of a season, a single individual may reproduce some millions simply by segmentation ; but 

 as cold weather approaches, we find the animal to lose this power — not resulting from the influence of the 

 cold, but from exhaustion of the power; because, even if the worms be placed in a warm situation, as in the 

 window of a warm room, where the sun may shine upon the vessel containing them, they are observed to 

 cease division. The loss of power of this mode of reproduction, is, however, compensated for by another 

 succession of developments. 



The worms grow to an inch in length and are composed of sixty annulations, each being provided with 

 double the previous number of podal spines. Within the body, aij androgynous generative apparatus be- 

 comes developed; within the ovaries are developed ova, and within the testis, spermatozoa. Two indivi- 

 duals copulate, eggs contained in bottle-shaped eases are extruded, and ultimately the parent dies. 



After some weeks, in a warm situation, the ova are hatched, the young escape and move freely about, and 

 soon commence to reproduce their numbers by division. 



