ALTERNATE GENERATION.
In several insect families, the species is not wholly represented in the adult individuals of both sexes, or in their development, but, to complete this series, supplementary individuals, as it were, of one or of several preceding generations, are required. The son may not resemble the father, but the grandfather, and in some instances, the likeness re-appears only in latter generations. Agassiz states: "Alternate generation was first observed among the Salpæ. These are marine mollusks, without shells, belonging to the family Tunicata. They are distinguished by the curious peculiarity of being united together in considerable numbers so as to form long chains, which float in the sea, the mouth(m) however being free in each.
"Fig. 2. The individuals thus joined in floating colonies produce eggs; but in each animal there is generally but one egg formed, which is developed in the body of the parent, and from which is hatched a little mollusk.
"Fig. 3, which remains solitary, and differs in many respects from the parent. This little animal, on the other hand, does not produce eggs, but propagates, by a kind of budding, which gives rise to chains already seen in the body of their parent(a), and these again bring forth solitary individuals, etc."
It therefore follows that generation in some animals require? two different bodies with intermediate ones, by means of which and their different modes of reproduction, a return to the original stock is effected.
Universality of Animalcular Life.—Living organisms are universally diffused over every part of the globe. The gentle zephyr wafts from flower to flower invisible, fructifying atoms, which quicken beauty and fragrance, giving the promise of a golden fruitage, to gladden and nourish a dependent world. Nature's own sweet cunning invests all living things constraining into her service chemical affinities, arranging the elements and disposing them for her own benefit, in such numberless ways that we involuntarily exclaim,
"The course of Nature is the art of God."
The microscope reveals the fact that matter measuring only 1/120000 of an inch diameter may be endowed with vitality, and that countless numbers of animalcules often inhabit a single drop of stagnant water. These monads do not vary in form, whether in motion or at rest. The life of one, even, is an inexplicable mystery to the philosopher. Ehrenberg writes: "Not only in the polar regions is there an uninterrupted development of active microscopic life, where larger animals cannot exist, but we find that those minute beings collected in the Antarctic expedition of Captain James Ross exhibit a remarkable abundance of unknown, and often most beautiful forms."
Even the interior of animal bodies is inhabited by animalcules. They have been found in the blood of the frog and the salmon, and in the optic fluid of fishes. Organic beings are found in the interior of the earth, into which the industry of the miner has made extensive excavations, sunk deep shafts, and thus revealed their forms; likewise, the smallest fossil organisms form subterranean strata many fathoms deep. Not only do lakes and inland seas abound with life, but also, from unknown depths, in volcanic districts, arise thermal springs which contain living insects. Were we endowed with a microscopic eye, we might see myriads of ethereal voyagers wafted by on every breeze, as we now behold drifting clouds of aqueous vapor. While the continents of earth furnishes evidences of the universality of organic beings, recent observations prove that "animal life predominates amid the eternal night of the depths of the liquid ocean."