xxxi A CHAPTER ON VIVIPARY 473 



ends with the detached immature seeds of many inland plants that 

 only germinate after lying for some time in the soil, is regarded as 

 supplying a record of the various epochs in the history of vivipary 

 throughout the plant-world. In the occasional cases of incomplete 

 vivipary occurring among inland plants and in the singular structure 

 presented by the seeds of certain genera of the Myrtaceae and other 

 orders we perceive indications of a lost viviparous habit belonging 

 to a primeval period when vivipary was the exception and not the 

 rule, an age when the same climatic conditions prevailed over much 

 of the globe. At such a period the sun's rays were screened off by 

 a dense cloud-covering that enveloped the earth, and the atmos- 

 phere was ever charged with moisture. With the differentiation of 

 climate that has marked the emergence of the continents during 

 the secular drying of the earth, the viviparous habit has been alone 

 retained within the confines of the mangrove-swamp, where the 

 conditions once almost universal now survive ; and as an adapta- 

 tion to the differentiation of climate and to the resulting seasonal 

 variation the rest-period of the seed has been developed. 



