Lamarck's theories. 85 



developed in a natural manner from previously existing, 

 different species; all are descendants of a single ancestral 

 form, or at least of a few common forms. The most ancient 

 ancestral forms must have been very simple organisms of 

 the lowest grade, and must have originated from inorganic 

 matter by means of spontaneous generation. Adaptation 

 through practice and habit, to the changing external condi- 

 tions of life, has ever been the cause of changes in the nature 

 of organic species, and Heredity caused the transmission of 

 these modifications to their descendants. 



These are the principal outlines of the theory of 

 Lamarck, now called the Theory of Descent or Transmuta- 

 tion, and to which, fifty years later, attention was again 

 called by Darwin, who firmly supported it with new proofs. 

 Lamarck, therefore, is the real founder of this Theory of 

 Descent or Transmutation, and it is a mistake to attribute 

 its origin to Darwin. Lamarck was the first to formulate 

 the scientific theory of the natural origin of all organisms, 

 including man, and at the same time to draw the two ulti- 

 mate inferences from this theory: firstly, the doctrine of 

 the origin of the most ancient organisms through spon- 

 taneous generation; and secondly, the descent of Man 

 from the Mammal most closely resembling Man — the Ape. 



Lamarck attempted to explain the latter process, a most 

 important one, and of special interest to us here, by the 

 same efficient causes to which he had also referred the 

 natural origin of animal and vegetable species. He con 

 sidered that, on the one hand, practice and habit (Adapta- 

 tion), and, on the other, Heredity, are the most important 

 of these causes. The chief modifications of the organs 

 of animals and plants result, according to him, from the 



