236 haeckel's anthrotogenie. 



The applicatiori of the theory to man is only a deduction from the 

 general inductive law of descent. This law is based on evidence 

 derived from different sources. Firstly, from palaeontology, we find 

 a gradual increase of number of species and a higher grade of evolu- 

 tion within the " type " reached as we ascend in the series ; secondly, 

 from comparative anatomy we find similarities of structure, on which 

 we base the natural system of classification; thirdly, from dystele- 

 ology in all higher animals we find rudimentary organs of no use to 

 their possessors; fourthly, from geographical distribution; fifthly, 

 the most important inductive proof, from embryology. 



The theoretical nature of species may now be considered definitely 

 settled. There are no limits between genus, species and variety^ 

 This is established by Haeckel's researches on the calcareous sponges' 

 which thus afford an analytical proof of the validity of the theory of 

 descent. It is necessary to have a clear conception of the nature of 

 the cell before entering on the study of ontogenesis, for eveiy animal 

 (and plant) consists at one time of its life of a single cell. Such a 

 cell is an independent organism, for it is capable of replying to a 

 stimulus, of movement, of nutrition and of reproduction. This view 

 of the individuality of the cell is expressed by classifying cells along 

 with eytodes as individuals of the first order (plastida). The cytode 

 differs from the cell in that its plasma is not yet differentiated into 

 nucleus and protoplasm. If the cell has a cell wall (as is i^arely the 

 case with animal cells), it is entirely an after-production, and is not 

 essential to the constitution of a cell. An organism may remain 

 unicellular, or it may form a commonwealth of individual cells, and 

 ^he great problem of ontogenesis is, " How is a multicellular developed 

 from a unicellular organism 1 " 



The essential characteristics of the eggs of all animals are the 

 same, as they are formed in the ovary of yolk, germinal vesicle and 

 germinal 'spot. In many lower animals they remain naked until 

 fertilized, and are thus amoeboid in form — sponges, hydroid polyjDes ; 

 indeed, they have been taken in sponges for parasitic amoebae. 

 Usually, however, there are special additions in the form of pro- 

 tective coverings or extra nutritive matter. The mammalian egg is 

 throughout the series about ^ m.m. in diameter, and is provided with 

 a covering — the zona pellucida — in which there are innumerable 

 porous canals. The bird's egg is, however, very different ; it is also 

 •unicellular, however large, but is provided with a complicated series 



