202 Causes and Course of Organic Evolution 



the fungus absorbs more than it gives, so that the flowering 

 host is steadily impoverished, till the individuals of a species, 

 and the species of a genus, and even the genera of a family, 

 slowly verge to extinction. 



Thus amongst Orcliidaceae the steady degeneration in species 

 of such genera as Goodyera, Aplectrum, Limodorum, Neottia, 

 Corallorhiza, and Epipogon, as they become increasingly sapro- 

 phytic; the like^ change in genera such as Pyrola, Chimaphila, 

 Alloiropa, Schweinitzia, and Monotropa amongst Ericaceae in 

 the "widest and only natural sense; that in Canscora, Obolaria^ 

 Bartonia, Voyria, and Leiphaimos amongst Gentianacese; and 

 the striking degeneration proceeding in all genera of the family 

 Burmanniacese; form a graded set of examples of the highest 

 value. 



Now while on the part of the symbiotic fungus, in each case 

 a definite environal stimulus was originally started, doubtless 

 between it and nutritive products of the host plant, that led 

 to proenvironal and invading response, the continued per- 

 sistence, extended invasion, and adaptation of the fungus, 

 as well as the gradual reduction to the stage of disappearance 

 of chlorophyll in the host; the reduced size and simplified 

 character in the stems and specially the leaves; as well as the 

 simplified flowers and embryos of the host plants; all furnish 

 perfect evidence of eliminating selection. 



Parasitism between one group of flowering plants that grad- 

 ually pass from a vigorous green to a simplified degraded and 

 ultimately colorless state and another group of flowering plants 

 that are parasitized on furnishes equally striking illustrations, 

 but cannot be enlarged on now. 



Sexual selection has been so fully debated that we need only 

 to recall it here. 



(5) Reproduction. The writer would now cite this as a 

 fifth and closing factor in evolution. It is one, however, that 

 has scarcely, if at all, been insisted on, by students of the sub- 

 ject, as a factor that deserves to be ranked with the preceding 

 four. But, if Darwin's famous aphorism "Nature abhors 

 perpetual self fertilization" be correct, then it might be expected 



