THE HISTORY OF ANIMAL LIFE 419 



possibility of a chance association of the substances in protoplasm 

 in the relation in which they now exist is not zero. Further than 

 this no statement can be made that amounts to anything more 

 valuable than a guess. 



A second question is also frequently asked, namely: Is new life 

 originating from the non-living now? So far as known, new living 

 forms are not now arising de novo; the only known source of liv- 

 ing material is other living material. Repeated attempts to demon- 

 strate the spontaneous origin of life have been made. In fact, 

 rumors of success appear with almost yearly frequency. None have 

 stood the test of critical examination. There is no hope of imme- 

 diate experimental production of the living protoplasm. The prin- 

 ciples that have been discussed on previous pages here have been 

 described rather than explained. Evolution and inheritance, repro- 

 duction and sex, development, growth, adaptation, contractility, irri- 

 tability and transmission, behavior and all the extraordinary powers 

 of the nervous system, are the end-products of reactions in the col- 

 loidal protoplasm. The various steps in these reactions are for the 

 most part as yet unknown. It is unlikely that we shall be able to 

 make something which we do not understand. 



A third question follows from the first and second, namely: Are 

 all living forms to be regarded as descendants of a single original 

 organism, or has life originated in numerous primitive types from 

 which are descended the multiplicity of present types .^^ And like 

 the first and second questions, the answer to this is also speculative. 

 In the days when biologists were heavily involved in searching for 

 facts having bearings on the validity of the theory of evolution, 

 these questions were worked over, and shaped the interests and 

 careers of the investigators. But now that the validity of the theory 

 of evolution has been established, these questions become of less 

 interest and importance; it is considered that any answer offered in 

 the light of our present fund of knowledge must contain a deal of 

 speculation. So no present-day biologist attempts a final answer. 



