ch. xxxi A CHAPTER ON VIVIPARY 469 



uniform climatic conditions of early geological periods, and that 

 with the differentiation of climates that has marked the emergence 

 of the continents the viviparous habit has been lost over much of 

 the globe, the mangrove-swamps alone illustrating the climatic 

 conditions once prevailing. The rest-period of the seed is 

 regarded as an adaptation to climatic differentiation and to 

 seasonal variation ; and even the seed-stage may be broadly 

 regarded as the price paid for adaptation on the part of the 

 evolutionary or determining power that lies behind plant-develop- 

 ment. When discussing the germination of Caesalpinia in Chapter 

 XVII, I have shown that the contraction and induration of 

 the seed-tests appear merely as an adaptation to climatic differ- 

 entiation and to seasonal variation, and that it would be quite 

 possible by exposing the maturing seed to very warm and moist 

 conditions to induce germination without any rest-period, as 

 actually occurs with Rhizophora. One would then dispense 

 altogether with the final processes of the contraction and indura- 

 tion of the seed-coats, as illustrated in the Leguminosae ; and the 

 rest-stage would appear as an adaptation to secular differentiation 

 of climate in the later epochs of the world's history. 



The significance of occasional vivipary was long ago pointed 

 out by Goebel in his Pflanzenbiologische Schilderungen (teil I., 

 1 17-134, Marburg, 1889), when he observed that vivipary, as 

 displayed in the mangroves, and particularly in the Rhizophoreae, 

 represented the fullest expression of a habit that is only occasion- 

 ally exhibited by other plants under exceptionally moist conditions. 

 His view was that the seeds of plants living in wet places are suited 

 in a varying degree for rapid germination, and that vivipary presents 

 itself as the most complete development of this capacity. If I 

 regard the views of Goebel and of Kerner aright, vivipary as 

 normally developed in the mangrove is to be traced in a descending 

 scale to small beginnings, the principal determining condition lying 

 in the great difference that exists amongst plants in the readiness 

 of the seed to germinate. In the ascending scale we would have 

 first the detachment of the immature seed, where the embryo is 

 often in a rudimentary state, the ripening of the seed taking place 

 in the soil. Then would come those plants where the seeds on 

 being detached are quite mature and are ready to germinate as 

 soon as they fall to the ground. Then would follow the stage 

 represented by those plants where the seeds merely begin to 

 germinate on the plant, such as occurs more or less normally with 

 some mangroves like Laguncularia, and abnormally with a number 



