344 THE CHANGING GENERATIONS 



Darwin himself accepted both Buffon's and Lamarck's theories as 

 contributing factors for evolution and often appealed to them to explain 

 evolutionary changes which he had difficulty in attributing to the opera- 

 tion of natural selection. Still other theories were advanced by various 

 post-Darwinians, some offered as supplements to and some as alternatives 

 for natural selection. All these theories are now largely abandoned and 

 forgotten or else (like orthogenesis) survive merely as descriptive terms 

 for certain real or supposed evolutionary sequences. 



None of the early work on evolution was either designed for or capable 

 of throwing light on the nature and limits of variation, inheritance, or 

 selection. To Darwin and his immediate successors these factors seemed 

 evident in nature and were appealed to in terms of logic and plausibility 

 to explain the facts of the past history of life. Then, in the period between 

 1880 and 1892, Weismann brought forward his concepts of the profound 

 distinction between germ and soma and of the continuity of the germ plasm. 

 As Weismann saw and stressed, here was a complete refutation of all 

 theories based upon the inheritance of acquired characters. Weismann 

 strongly championed selection (in conjunction with variation and in- 

 heritance) as the sole cause of evolution. Much of his work was highly 

 theoretical; but his distinction between germ and soma and his concept 

 of the continuity of the germ plasm were based upon good though not 

 extensive experimental evidence and were destined to be further sub- 

 stantiated by subsequent workers. Nevertheless Weismann's ideas were 

 dismissed or ignored by many biologists, and particularly by the paleon- 

 tologists. As students of the fossil record the paleontologists felt that they 

 had strong and convincing evidence, even though it was indirect and 

 circumstantial, to prove that acquired characters had been and are 

 inherited. 



We have not space to describe the long, interesting, and often bitter 

 controversies that followed Weismann's distinction between Darwinian 

 and Lamarckian factors, nor can we outline the various alternative or 

 subsidiary theories of evolution that were advanced in the period be- 

 tween 1890 and 1910. Biologists were, however, becoming increasingly 

 aware that they had little more positive knowledge of the processes of 

 evolution than had Darwin and his contemporaries and that they needed 

 far more exact information concerning the details and limits of variation, 

 inheritance, and selection. 



It was only when investigators shifted their point of attack that rapid 

 progress in understanding evolution began to be made. Instead of trying 

 to decipher the events and processes of the past, many biologists under- 

 took to study evolution in living organisms, using experimental methods 

 as much as possible. This phase of evolutionary biology is often described 

 as the study of speciation, to denote its concern with the relatively minor 



