190 VARIATION AND HEREDITY 



tion plays an important part in the formation of new species. Lichens, however, 

 are organisms which maintain themselves indefinitely by asexual means 

 without any intimate union of the fungus and alga occurring. It is. however, 

 possible that pronounced formative or other changes might result from 

 intraccllular symbiosis, such as is shown between certain algae and Hydra 

 viridis or Radiolaria. In these cases the algae reproduce themselves, and 

 are transferred to the offspring, so that the symbiosis is permanent. If 

 a bacterium or amoeba entered into symbiotic union with a protoplast, and 

 lost the power of separate existence, it would appear to us to be as much an 

 integral protoplastic organ as are the chloroplastids which maintain them- 

 selves by independent division 1 . Further, a new species would probably be 

 produced if a foreign nucleus and cytoplasm could be caused to enter into 

 permanent union. 



All variations which are not produced by symbiogenesis must be 

 produced by some change in the organism itself. If this is indirectly due 

 to peculiar external conditions, the variation may be termed aitiogenic, but 

 autogenic if it arises spontaneously. Such changes need not necessarily 

 involve an actual alteration in the germ-plasma, for the appearance of some 

 peculiar stimulating metabolic product which was able to excite its own 

 renewed formation and transference to the offspring, might produce a 

 permanent hereditary variation. 



The changes of shape due to symbiogenesis do not necessarily involve 

 any change in the character of the germ-plasm 2 , for on isolation each 

 symbiont follows its original life-cycle. It is often the case that variations 

 may be again slowly deleted by retrogressive modification, whereas others 

 seem to be permanent, although, as in existing species, retrogression or 

 further modification might take place under special conditions. 



Many cultural varieties can only be propagated vegetatively, for sexually 

 produced offspring tend to revert to the parent stock. In connexion 

 herewith, it would be of importance to know whether the primary meristems 

 have inherent in them the characters of the cultural variety, or whether 

 these characters are impressed upon the differentiating cells by the pre- 

 formed ones. To answer this question it would be necessary to develop 

 an entire plant from a single meristem cell, isolated, and hence removed 

 from the influences exercised by adult tissues. Certain facts do, however, 

 seem to indicate that the primary meristem, at least in some cases, has the 

 characters of the race or variety inherent in it. In this case a retrogressive 



Centrnlbl., 1900, Bd. LXXXIV, p. 97; de Vries, Bot. Ztg., 1900, p. 435. [Zenia is the name given 

 when some peculiarity of the pollen-bearing parent makes itself manifest in the seed. Cf. Correns, 

 Bastarde zwischen Maisrassen mit besonderer Beriicksichtigung der Xenien, Bibl. Bot., 1901.] 



1 Certain diseases are apparently transferred to the offspring by the entry of the parasitic bacteria 

 into the germinal cells. 



2 Cf., among others, Ortmann, Biol. Centralbl., 1898, Bd. xvnr, p. 142. 



