270 the president's address. 



amoeboid form results the perfect imago, or image of the plant from 

 which originally the ovum was derived.* If exception should be 

 taken to any of the Myxogasters being employed in illustration, 

 inasmuch as their vegetable nature has been called in question, then 

 we can fall back on the life history of Volvox globator, Stephano- 

 sphcera, and other of the Volvocinece, to say nothing of mosses and 

 Characece, already alluded to, which furnish less perfect transforma- 

 tions. Although not conducted on so large a scale as in the animal 

 kingdom, it is clear that we have at least suggestions of metamor- 

 phosis also in the vegetable world. 



Alternation of generations, as applied zoologically, differs 

 materially from metamorphosis, although they are sometimes con- 

 founded as though they were convertible terms. The fundamental 

 idea is that of an organism " producing an offspring which at no 

 time resembles its parent, but which, on the other hand, itself brings 

 forth a progeny which returns, in its form and nature, to the parent 

 animal, so that the material organism does not meet with its resem- 

 blance in its own brood, but in the descendants of the second, third, 

 or fourth degree or generation, and this always takes place in the 

 different animals which exhibit the phenomenon in a determinate 

 generation, or with the intervention of a determinate number of 

 generations. "f The characteristic difference between this and a 

 simple metamorphosis is that each generation completes its career in 

 the same form as it commenced, so that each starts from an ovum, 

 and the cycle is not the career of a single individual, but of a con- 

 secutive series of individuals, which revert to the original form after 

 one, two, or more intermediate and differing generations. 



In Ferns an alternation of generations is evident. The fronds of 

 mature ferns bear on their under surface, or margin, clusters of spore- 

 cases containing minute spores, which themselves are produced 



* When placed in water the membrane of the spore opens, and its con- 

 tents escape in the form of a cell clothed only by a very thin primordial 

 utricle. These escaped cells undergo changes of form, eventually exhibiting 

 one or two cilia. They have also a motion of progression and rotation, as in 

 the case of ordinary zoospores. After a few days bodies appear precisely 

 as in the amoeba, like which they have a creeping motion, and are per- 

 petually changing their form. These are produced from the zoospores. 

 From these again the author proceeds to show the complete form of tli3 

 Myxogaster is developed, concluding, " direct development of the fructifying 

 threads from the Amoeba, produced by the growth of the zoospores, ap- 

 pears tome beyond a doubt." Sec " Qnart. Journ. Micr. Science," I860, p. 

 100. 



f Steenstrup, "Alternation of generations, '" Kay. Society, p. 1. 



