416 EVOLUTION AND GENETICS 



which the parents had acquired. Such structures as heritable 

 callosities are obviously similar to the calluses which can be 

 developed in an individual, but relationship between the two has 

 not been proved. The association of characters of these two types 

 has long been one of the most vexatious problems in biology, and 

 has been the foundation for many experimental studies. 



Necessity in the Production of Organs. The idea that need 

 in an organism may bring about the development of a new organ 

 is scarcely worthy of scientific attention. On a purely logical 

 basis it may be refuted, for organisms, in order to exist, must 

 have all of the organs necessary for successful existence. In 

 human experience need may be interpreted in a different way; 

 in the biological sense only those things are needed which enable 

 the organism to live successfully and to perpetuate the species. 

 In an examination of the facts of biology, and in particular of 

 adaptation, it seems rather that organisms have made use of such 

 things as they have possessed than that they have developed 

 organs to meet preexisting needs. 



The Unity of the Organic World. Finally Lamarck's opinion that 

 the environment acts directly on plants and indirectly through 

 the nervous system on animal structures expresses a naive disregard 

 for the fundamental unity of the organic world which could hardly 

 fail to weaken the theory of which it is a part. A liberal inter- 

 pretation shows that it is probably based on the much more inti- 

 mate association of plants with the physical environment. We now 

 recognize that whatever is true of the fundamental evolutionary 

 processes of one kingdom is in all proba])ility true of the other. 



Lamarck's theory therefore narrows down to one critical point 

 of controversy, viz., the inheritance of acquired characters. All 

 other premises are either well established or universally denied, 

 but this one point has baffled scientists. Of the many attempts 

 which have been made to prove or disprove it experimentally a 

 few are significant. The results obtained have generally failed to 

 establish even the possibility of such inheritance, but positive 

 disproof has also been lacking. The question still arises and still 

 finds ardent defenders as well as opponents. 



Acceptance of the Inheritance of Acquired Characters. The 

 proponents of Lamarck's view include all kinds of thinkers, among 

 them men of philosophical mind who were content with a purely 

 logical analysis of the question and able biologists whose opinions 



