390 PROVISIONAL HYPOTHESIS Chap. XXVIL 



should he far less variable than those propagated sexually. 

 With respect to the direct action of changed conditions, we 

 know that organisms produced from buds do not pass through 

 the earlier phases of development; they will therefore not 

 be exposed, at that period of life when structure is most 

 readily modified, to the various causes inducing variability 

 in the same manner as are embryos and young larval forms ; 

 but whether this is a sufficient explanation I know not. 



With respect to variations due to reversion, there is a 

 similar difference between plants propagated from buds and 

 seeds. Many varieties can be propagated securely by buds, 

 but generally or invariably revert to their parent-forms by 

 seed. So. also, hybridised plants can be multiplied to any 

 extent by buds, but are continually liable to reversion by 

 seed, — that is, to the loss of their hybrid or intermediate cha- 

 racter. I can offer no satisfactory explanation of these facts. 

 Plants with variegated leaves, phloxes with striped flowers, 

 barberries with seedless fruit, can all be securely propagated 

 by buds taken from the stem or branches ; but buds from the 

 roots of these plants almost invariably lose their character 

 and revert to their former condition. This latter fact is also 

 inexplicable, unless buds developed from the roots are as 

 distinct from those on the stem, as is one bud on the stem 

 from another, and we know that these latter behave like 

 independent organisms. 



Finally, we see that on the hypothesis of pangenesis varia- 

 bility depends on at least two distinct groups of causes. 

 Firstly, the deficiency, superabundance, and transposition of 

 gemmules. and the redevelopment of those which have long 

 been dormant : the gemmules themselves not having under- 

 gone any modification ; and such changes will amply account 

 for much fluctuating variability. Secondly, the direct action 

 of changed conditions on the organisation, and of the increased 

 use or disuse of parts ; and in this case the gemmules from 

 the modified units will be themselves modified, and, when 

 sufficiently multiplied, will supplant the old gemmules and 

 be developed into new structures. 



Turning now to the laws of Inheritance. If we suppose a 



