DEVELOPMENT OF ZOOLOGY 515 



behavior of the cellular elements therein; the nature of the 

 chromosomes and their part in heredity; the effect of external 

 agencies upon development; the origin of the special types of 

 tissues ; the continuity of the germ plasm and even of germ cells 

 in the cycle of generations. 



520. Philosophy of Biology. In the early periods of human 

 thinking the theories outran the facts. During all the time 

 much philosophy has accompanied the discoveries. So all the 

 great names that have been mentioned have contributed some- 

 thing to our philosophy of Biology. Important as the field 

 is we can here only mention a few salient steps. Some of 

 them have already been suggested. The nature of some of 

 them have been more completely discussed in Ch. XXVI. These 

 problems include the origin and nature of living things, the 

 causes that have operated to bring them to their present state, 

 and the principles that seem to underlie the process. 



It may be said that two main theories obtained among the 

 ancients to explain life as we have it on the earth. One held 

 that organisms were created supernaturally much as they are 

 at present, closely suited to the environment in which they 

 live. The other considered that in some way the present is a 

 natural development from the past, and that organisms have 

 grown into their various adjustments to external conditions. 

 It is impossible to say how early this latter idea of evolution, so 

 generally held at present, originated. We find evidences of it 

 before Aristotle and still more in his writings. 



Theoretically, the following views of life might be held : 



1. That life has always been on the earth, and only life 

 can give rise to life. 



2 . That at some time life appeared on the earth, from non-life, 



a. Suddenly, much as it is at present. 



b. Gradually, starting in simple form and becoming 

 complex as at present. 



Two fundamental questions are here : Did life start de novo at 

 some time? Was it constant after it started? Another has 

 arisen since: Can life start anew at present (spontaneous 

 generation) ? 



