HISTORY OF ZOOLOGY 35 



tingmshing the larger groups. Linnaeus believed in a rigidly fixed 

 species and had divided the animals into six classes, 32 sub-classes, 

 and numerous genera and species. In spite of his idea of the in- 

 variability of species his classification system was so simple, clear, 

 and flexible that it has persisted to the present time. His was the 

 first natural system of classification, and it is known as the Binomial 

 System of Nomenclature. Each individual not only fits into larger 

 general groups by this system, but it is specifically known by the 

 genus name and species used together, hence the two names. Lin- 

 naeus is said to have classified and listed 4,378 species of plants and 

 animals. 



Almost immediately following Linnaeus came the Frenchman, 

 Lamarck (1744-1829), who among other important things is credited 

 with being first to realize that there are different lines of descent 

 and that no living species is absolutely fixed. Much later, in 1866, 

 Ernst Haeckel organized the modification of this system as used in 

 modern times. 



Georges Cuvier (1769-1832) is credited with establishing the field 

 of comparative anatomy. He was of French ancestry and largely 

 self-educated by his studies at the seashore. A number of anatomical 

 structures bear his name. 



Karl Ernst von Baer (1792-1876), a Russian biologist, is one who 

 really established embryology as a field of study. His notable paper 

 on the development of the chick was published in 1832. He estab- 

 lished the ''germ layer theory," thus explaining the unfolding and 

 differentiation of the various organs of the developing animal. The 

 recapitulation theory, which is explained elsewhere, came as a result 

 of his work and thought. 



Johannes Miiller (1801-1858), a German scientist, is referred to 

 as the founder of comparative physiology and the first to apply the 

 facts of physics and chemistry to living protoplasm. His work was 

 a great impetus to modern physiology. 



Matthias Schleiden (1804-1881) and Theodor Schwann (1810-1882) 

 are the two Germans who in 1838-1839 arrived at one of the most 

 important generalizations of biology, the cell theory (principle). 

 This is to be discussed further in the following chapter. 



Louis Agassiz (1807-1873) is commonly regarded as the father of 

 American zoology and a renowned student of comparative anatomy. 

 His great inspiration has permeated through his students to nearly 



