278 Transactions. — Botany. 



development of an individual a certain phase occurs, the 

 more recently must the condition that the phase in question 

 represents have been the fixed form of the species. Professor 

 W. F. Ganong goes into this matter at some length and with 

 great clearness.* " Of course, in any given generation the 

 embryo, \ apart from slight irritable responses to light, &c, is 

 determined by heredity. But heredity is but the sum and 

 resultant of past experiences, and hence in the present case is 

 largely a study of past environments. This suggests an ex- 

 planation which I believe to be the true one — i.e., that the 

 form of the adults, like that of any other character once 

 acquired — it matters not for our present purpose how — as it 

 becomes more and more fixed and intensified tends to work 

 back into earlier and earlier stages in the ontogeny of the 

 successive individuals, until finally a character adaptively ac- 

 quired by the adults works back into the epicotyl and finally 

 into the embryo itself." Further on he writes " of characters 

 acquired by adaptation in the adults sweeping back into the 

 later seedlings and wiping out earlier characters." Accord- 

 ing to the above that semi-shrubby form of Plagianthus betu- 

 linus of New Zealand, which remains for a long time in 

 the ontogeny of that plant as a seemingly adult form, and, 

 moreover, persists for a long time even after the real adult 

 form has appeared, must have been the final form of the 

 plant at no very distant date, geologically speaking. But it 

 cannot have remained a fixed form for any great length of 

 time, for it did not work back into the epicotyl, and so wipe 

 out the earlier stage which so much resembles the final one. 

 In the Chatham Island plant, on the other hand, the first 

 seedling form being so little different from the final shows 

 that the latter has endured for a very long time, nor, so far 

 as the first form tells us, has ever undergone much change 

 since it first became fixed. But the first seedling form of the 

 Chatham Island and of the New Zealand plant are very 

 similar, and so we can well conceive that a plant almost iden- 

 tical with the Chatham Island form was the ancestor of the 

 New Zealand form. The same reasoning would apply to- 

 Sojjlwra tetraptcra (using the specific name in its widest 

 sense), which is even a more instructive species. For one 

 variety of that species not only goes through three forms in 

 the course of its ontogeny, each much resembling in habit 



* "Annals of Botany," 1898, vol. xii., "Contributions to a Know- 

 ledge of the Morphology and (Ecology of the Caotacese : II. The Com- 

 parative Morphology of the Embrjos and Seedlings," pp. 467, 468. 



f " We may best speak of the stage where the embryo is lying in the 

 seed as that of the ungerminated embryo, and that in which it has come 

 out, turned green, spread its cotyledons, but before it shows the epicotyl, 

 as that of the germinated embryo " (Ganong, I.e., p. 431). 



