350 SCIENTIFIC THOUGHT. 



time also no attempt was made to bring phytogenesis 

 the genesis of plant-life into line and order with 

 zoogenesis, the genetic arrangement of animals. It is 

 Haeckel's undoubted merit to have attempted for the 

 first time to carry out this general scheme on a large 

 scale, and by means of detailed pedigrees ; beginning 

 with the undefined organisms in which as yet the 

 peculiar characters of animal- and plant - life do not 

 appear to be differentiated, and ascending in two great 

 trunks into the vegetable and animal kingdom, and 

 thence through many ramifications into the several 

 .classes, families, genera, species, and varieties of living 

 things, to construct the supposed real natural system 

 for which systematists had been unconsciously searching 

 since the age of Eay and Linnreus. For the purpose of 

 elaborating this great scheme he employs not only the 

 great law of heredity, according to which ancestral 

 characters are reproduced in development, but also the 

 older law of adaptation to the existing environment, as 

 45. pointed out by Lamarck. Haeckel, in fact, combines the 

 Darwin and views of Darwin and Lamarck, which other naturalists 



Lamarck. 



are more or less inclined to keep apart, whence has 

 arisen the well-known division into the two great schools 

 of the neo-Darwinians and neo-Lamarckians. 1 Though 



1 Natural selection being an ad- 

 mitted fact among living things, 

 like gravitation in the physical 

 universe, three distinct problems 

 arise : First, how far does it reach ? 

 the scope of the principle. The 

 subsequent writings of Darwin were 

 mainly occupied with this question, 

 though as we shall see later he 

 also ventured upon an important 



suggestion as to the underlying 

 problem of inheritance. Secondly, 

 the fact or principle itself requires 

 to be traced to deeper-lying causes. 

 We may say natural selection is a 

 vera causa, but not a prima causa : 

 it is a true but not a prime cause. 

 The investigations regarding "varia- 

 tion " and "heredity" work along 

 this line of research, and form the 



