Simplified Statement of Lamarck's Views.--For practical exposition the theory maybe simplified into two sets of facts: First, those to be classed under variation; and, second, those under heredity. Variations of organs, according to Lamarck, arise in animals mainly through use and disuse, and new organs have their origin in a physiological need. A new need felt by the animal expresses itself on the organism, stimulating growth and adaptations in a particular direction. This part of Lamarck's theory has been subjected to much ridicule. The sense in which he employs the word _besoin_ has been much misunderstood; when, however, we take into account that he uses it, not merely as expressing a wish or desire on the part of the animal, but as the reflex action arising from new conditions, his statement loses its alleged grotesqueness and seems to be founded on sound physiology. Inheritance.--Lamarck's view of heredity was uncritical; according to his conception, inheritance was a simple, direct transmission of those superficial changes that arise in organs within the lifetime of an individual owing to use and disuse. It is on this question of the direct inheritance of variations acquired in the lifetime of an individual that his theory has been the most assailed. The belief in the inheritance of acquired characteristics has been so undermined by experimental evidence that at the present time we can not point to a single unchallenged instance of such inheritance. But, while Lamarck's theory has shown weakness on that side, his ideas regarding the production of variations have been revived and extended. Variation.--The more commendable part of his theory is the attempt to account for variation. Darwin assumed variation, but Lamarck attempted to account for it, and in this feature many discerning students maintain that the theory of Lamarck is more philosophical in its foundation than that of Darwin. In any theory of evolution we must deal with the variation of organisms and heredity, and thus we observe that the two factors discussed by Lamarck are basal. Although it must be admitted that even to-day we know little about either variation or heredity, they remain basal factors in any theory of evolution. Time and Favorable Conditions.--Lamarck supposed a very long time was necessary to bring about the changes which have taken place in animals. The central thought of time and favorable conditions occurs again and again in his writings. The following quotation is interesting as coming from the first announcement of his views in 1800: "It appears, as I have already said, that _time_ and _favorable conditions_ are the two principal means which nature has employed in giving existence to all her productions. We know that for her time has no limit, and that consequently she has it always at her disposal.