WHAT IS A SPECIES? 159 



the general theory of Evolution, relates to the manner, or 

 methods, or means by which living organisms are developed, 

 or evolved, from one another; namely, the inherent suscepti- 

 bility and tendency to variation according to conditions of 

 environment ; the preservation and perfection of organs best 

 suited to the needs of the individual in its struggle for exist- 

 ence ; the perpetuation of the more favorably organized 

 beings, and the destruction of those less gifted to survive; 

 the operation of natural selection, in which sexual selection is 

 an important factor; and the general proposition that at any 

 given time any given organism represents the result of the 

 foregoing factors, acting in opposition to the hereditary ten- 

 dency to adhere to the type, or ' breed true 



The Lamarckiaii Theory of Evolution, — " The portion of the 

 theory of Development [Ex'olution] which maintains the com- 

 mon descent of all species of animals and plants from the 

 simplest common original forms might, therefore, in honor of 

 its eminent founder, and with full justice, be called Lauiarck- 

 ian ; on the other hand, the theory of Selection, or breeding, 

 might be justly called Darwinism, being that portion of the 

 theory of Developm.ent [Evolution] which shows us in what 

 Avay, and why, the different species of organisms have de- 

 veloped from those simplest primary forms."* 



Phylogenetic Evolution. — We may quote again from the 

 Century Dictionary the definition of PJiylogcnctic Evolution : 

 " It is the name for that form of the doctrine of Evolution 

 which insists upon the direct derivation of all forms of life 

 from other antecedent forms, in no other way than as, in 

 ontogeny, offspring are derived from parents, and conse- 

 quently grades all actual affinities according to propinquity, 

 or remoteness of genetic succession. It presumes, as a rule, 

 such derivation or descent, with modification, is from the 

 more simple to the more complex forms, from low to high in 

 organization, and from the more generalized to the more 

 specialized in structure and function ; but it also recognizes 

 retrograde development, degeneration or degradation." 



The law of Evolution is put in a terse form by Huxley, who 



* Haeckel, " Hist, of Creation," etc., vol. i. p. 150. 



