differ more or less from one another in shades of color, but their valuable qualities appear in the increased solidity or consistence, size and fine flavor of the fruit. Their origination has, in many cases at least, been sudden and fortuitous; and, although their specific characters are usually heritable, they are so peculiarly subject to degeneration that they hardly possess racial properties. The causes of their origination and extinction appear to be irrelative, and both are independent of specific plant mutation. Still, as I shall show, a few cases of coincidence of their origin with plant mutation are known; and more or less of common plant variation, and even of dwarfing, have sometimes occurred coincidently. Moreover, all cases of atavic reversion, or degeneration, of fruit varieties from their fine quality have apparently been unaccompanied by any material change in the plants bearing the degenerating fruit. Frequently special reference to those reversions will be made further on, but I will first turn aside for a few more or less theoretical remarks upon the character and manner of origination and extinction of the fruit varieties.
In the simple cases of origination of the fruit varieties of the tomato, unaccompanied with plant mutation, the change occurs only in the pericarp, which becomes the fruit. Also, when atavic reversion occurs it is only the pericarp, that is, the fruit, which is materially affected; and both kinds of these changes are doubtless of molecular origin in the germ cells. Likewise, in all cases of phylogenetic plant mutation the initial act is molecular, and occurs in the germ cell of each ovule which gives origin to a new plant form.[1] There is no apparent reason to doubt that the mutative acts which thus respectively produced the two new specific forms from L. esculentum might have occurred without the coincident production of a new fruit variety, but as a matter of fact a new fruit variety of high quality was coincidently produced in each case; and the coming change in both plant and fruit was doubtless initiated in one and the same germ cell. Still, because of the essential difference between plant mutation and fruit variation, I think this pericarpal change was only a varietal coincidence, and not an essential part of the phylogenetic process in those cases. Moreover, that change is known to have occurred separately in other cases, and to have resulted only in weakly heritable fruit varieties.
Known cases of degeneration, and final extinction of fine fruit varieties as such, are somewhat numerous, for their instability is very great, although at least most of them remain true to seed from year to year under favorable conditions. Whether the varieties which have
- ↑ For exhaustive discussions of this and kindred subjects see 'Intracellular Pangenesis' by Hugo de Vries. Jena, 1889; and also 'Die Mutationstheorie,' vol. 1, by the same author.
