Jean-Baptiste Lamarck
(1744-1829)

Although Lamarck was not the first person to propose the idea of evolution, he was the first person to propose a method for the working of evolution.   He also believed in directed evolution, where simple organisms strive to become more complex, with humans being the ultimate goal. 

I. Lamarck's proposal on how evolution occurs consisted of two main parts:

1. Principle of use and disuse--

2. Principle of inheritance of acquired characteristics-
Traits or characteristics acquired during the lifetime of an organism can be passed on to its offspring.


II. Problems with Lamarck's Principle:
Lamarck's Principle of use and disuse is generally considered to be on target, but his Principle of inheritance of acquired characteristics is simply not true.  We now know that hereditary characteristics are passed from one generation to the next via genes/chromosomes, but this was not known during Lamarck's time. 
According to natural selection, therefore, parts which are important adaptations, and enable the organism to have a better chance at survival and reproduction, will be selected for; whereas, those parts which are deleterious to survival will be selected against; and those parts which are neither beneficial nor harmful, will have no selective value.  This generally supports Lamarck's ideas about use and disuse.
Lamarck explained the evolution of the long necks of giraffes as having occurred by the stress and strain of reaching their necks high into trees for food.  Each generation would reach higher, causing the next generation to have longer necks.  Of course, we now know that the real mechanism for the longer necks is natural selection: that those giraffes with long necks who could reach the food had a better chance of surviving and producing offspring with long necks, than those giraffes with short necks.

III. August Weissman
August Weissman tested Lamarck's Principle of Inheritance of Acquired Characteristics by cutting off the tails of mice and breeding them to see if their offspring were tail-less.  After cutting the tails off of 57 generations, Weissman reported that none of the offspring were without tails.