GENERAL CONSIDERATIONS. 553 



excreted by different gall-producing insects are specifically distinct. It is only in 

 this way that we can account for the fact that the same vegetable protoplasm is 

 incited in one case to produce a fleshy covering gall, in another a hollow pocket, and 

 in a third a closed gall-apple as dwellings for the particular insects concerned. 



It should also be mentioned that the same species of insect produces very similar 

 but slightly different galls on different plants. For example, the gall produced 

 by Nematus pedunculi on the lower side of the white-haired leaves of Salix 

 incana is covered with a white felt of hairs, that which the same gall-gnat pro- 

 duces on the smooth leaves of Salix purpurea is smooth; the gall produced by 

 Rhodites Rosce on the light green leaves of Rosa canina is pale yellow and some- 

 what reddened on that side turned towards the sun; that on the violet leaves of 

 Rosa rubrifolia produced by the same insect-species is dark violet, &c. These 

 distinctions, though only insignificant, show how certain external characteristics 

 founded in the specific constitution of the protoplasm of different plant-species find 

 expression even in the gall-structures. 



These facts confirm the view that the fluids excreted by different species of 

 insects, as well as the protoplasm of each plant species, have a peculiar composition. 

 It is then quite obvious that the alteration which the protoplasm of a species of 

 plant undergoes under the influence of a specific fluid will be subject to definite 

 laws. The protoplasm of the particular plant-cell receives by reason of the 

 alteration, as it were, a new definite constitution with tendencies not the same 

 as before; but since this constitution determines the outer form of the tissue 

 derived from these cells, the tissue itself will become shaped into a particular specific 

 form. These conclusions are of importance with respect to the question of the 

 origin of new species, inasmuch as they throw some light on the processes which 

 lead to the origin of new forms. We can now say that the alteration in the form 

 of a plant only occurs if the constitution of the protoplasm which forms the starting- 

 point of the plant is itself first altered. 



The structures known as galls have not the power of maintaining and multi- 

 plying themselves, but when their task is ended they perish. In other words, the 

 progeny arising from the seeds of a plant beset with galls exhibits none of the 

 alterations shown by the members or shoots of the parent plant. If, for example, 

 an Oak which is covered with galls is propagated by seeds, the offspring show 

 no trace of the structural alterations exhibited by the branches, foliage, or flowers 

 of the mother-plant. The only change which is perhaps sometimes retained 

 in the offspring is the metamorphosis of the stamens into petals, which has long 

 been known as doubling, and perhaps also the formation of fasciations, &c. in the 

 floral region, as in Cabbages (where it is known as a Cauliflower). Few attempts 

 have yet been made to investigate this matter. My own knowledge of the subject 

 is restricted to some observations made on the Speedwell Veronica officinalis. 

 Plants of Veronica officinalis which in consequence of the settlement of gall-mites 

 on them produced double flowers in 1877 in the garden of my country house were 

 planted close beside others free from gall-mites and with normal flowers. In the 



