80 THE QUANTITATIVE METHOD IN BIOLOGY 



a previous state. This ontogenetic method had an enormous 

 influence upon the progress of morphological science, apart 

 from any theory about the origin and phylogenetic relations 

 of species.! The results obtained by the comparative method 

 which was formerly used alone {Comparative Anatomy of 

 CUVIER), and which is based upon the comparison of adult 

 specimens, have been explained, verified and often corrected by 

 embryological investigation. 



It may be asked, however, whether the universal use of the 

 embryological method does not divert our attention from 

 certain aspects of the morphological problems. 



In the example of Pediastrum the curious structure of the 

 adult specimen is quite independent of any previous state. It 

 appears suddenly, by the juxtaposition of parts (cells) between 

 which there was previously no relation at all. Since our 

 morphological (embryological) method does not afford any 

 explanation of the structure produced in this way, Pediastrum 

 is put apart as a ciuiosity. 



Pediastrum is not an isolated case. It is impossible to find 

 any embryological explanation of the regular disposition of the 

 cells in an adult specimen of Volvox, Hydrodictyon, etc. (See 

 § 68.) Other interesting examples are found among the Myxo- 

 mycetes. In the numerous species of this group a certain 

 number of isolated cells (amaebo-spores, etc.) are produced by 

 successive divisions of one initial cell (spore). These cells 

 wander about within the substratum ; after a certain time two 

 cells meet each other and are united into a Plasmodium, which 

 becomes a centre of attraction : more cells join this centre. In 

 the Plasmodium no definite morphological structure exists 

 because the relative position of the cells (represented by their 

 nuclei) is continually modified by ameeboid movements. At a 

 certain moment the movements are stopped and the society of 

 cells is brought into a state of stable equilibrium ; it becomes 

 transformed into a so-caHed fruit. 



In numerous Myxomycetes the fruit has a regular form which 

 is characteristic of each species. Since it is impossible to find 

 any relation between this form and the preceding state, morphol- 

 ogy based on embryology gives us little help when we want to 

 investigate the structure of the fruits of the Myxomycetes. 

 Here a morphology of a peculiar kind ought to be created. The 

 primordia of the objects under consideration ought to be deter- 

 mined and measured in order to find a base for comparison. 

 Each primordium represents a state of equilibrium or, in other 

 words, a part of a system which is in equilibrium. An exact 



1 A similar method is applied in historical science ; for instance, in palaeog- 

 raphy, philology, numismatics (see the works of LELEWELL, SERRURE 

 and others), economic history, etc. 



