120 DUDLEY MEMORIAL VOLUME 



Godetia, of CEnothera, and Opuntia. Eucalyptus, Acacia and Epacris in 

 Australia axe examples even more striking. But I have never seen very 

 closely related or geminate forms in any of these genera actually growing 

 together. I suspect that they do so sometimes and that the explanation is 

 found in reinvasion. Dr. G. H. Shull of the Carnegie Station for Experi- 

 mental Evolution tells me that most of these plants are self-fertilized, a con- 

 dition unfavorable to Panmixia or the loss of the individual or local varia- 

 tion in the mass. Self-fertilized plants may be neighbors without really 

 "growing together." But "growing together" is an indefinite statement as 

 applied to plants. The elder, the alder and the loadroiio (Arbutus) abound 

 in the Santa Clara Valley. But no one ever saw any two of these trees stand- 

 ing side by side. Each has its limitations, as to soil and moisture, and its 

 own choice of locations. 



Setting aside these genera which are represented by many species in a 

 limited area, and among which mutation, hybridism and self-fertilization may 

 be conceivable factors in species-forming, we find the law of geminate species 

 applying to plants as well as to animals. Crossing the temperate zone any- 

 where on east and west lines, we find species after species replaced across 

 the barriers by closely related forms. Illustrations may be taken anywhere 

 among the higher plants — equally well, no doubt, among lower ones. Many 

 genera are local in their distribution, monotypic — with a single species, the 

 origin of which cannot be traced. Such species spread far and wide without 

 visible change within the species. 'But many other genera belt the earth or 

 come very near doing so, each form or species being geminate as related to 

 its next neighbor. This fact is illustrated in Rubus, Alnus, Sambucus, Pla- 

 tanus, Fagus, Veratrum, S5miplocarpus, Symphoricarpus, Castanea, Quercus, 

 Pinus, Tsuga, Acer, Rhus, Pyrus, Prunus, Lonicera, Ranunculus, Trientalis, 

 Lilimn, Trillium, Veronica, Aquilegia, Gentiana, Viola, Epilobium, Pteris, 

 Mimulus, Trifolium, Solidago, Aster, Helianthemum, Triosteum, Geranium, 

 Ribes, Asarvmi, Habenaria, SaxSfraga, Clintonia, Calycanthus, Fraxinus, 

 Philadelphus, Crataegus, Azalea, Erythroniimi, Rhododendron, Viburnum, 

 Comus, Cercis, Eupatorium. All these genera and many others furnish an 

 abundance of examples. It would be hard to find a widely-distributed genus 

 which did not. 



Taking a single example, the pink-flowering raspberry of the eastern 

 United States, Rubus odoratus, becomes on the Pacific Slope the white-flow- 

 ered Rubus parviflorus, ("nutkanus"). On the California sea coast, Rubus 

 velutinus, with tasteless fruit, again takes the place of the latter. The black 

 raspberry, Rubus occidentalis, is replaced westward by its double, Rubus leu- 

 codermis. The common blackberry, Rubus villosus, is replaced in the eastern 



