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64 THE QUANTITATIVE METHOD IN BIOLOGY 



by external causes; that the characteristics (combination 

 primordia) of an embryo in a given state may be therefore 

 modified, bringing about a new starting-point, and that the 

 further development may be modified in its turn. 



We have seen, for instance (§ 47, p. 56) that in Primtda 

 sinensis an original transitory primordium (white) is trans- 

 formed by unusual temperature into an original arrested per- 

 sistent primordium, a second primordium (red) being sup- 

 pressed altogether, while a third primordium (length) is hardly or 

 not altered. There is, a priori, no reason why similar changes 

 should not occur in the course of the individual develop- 

 ment in any species whatever. It is only by means of the 

 quantitative method (curves of development) that it is possible 

 to investigate accurately such phenomena. (Compare Part 

 VIII. , gradation curves.) 



§ 51. — REMARK : The quantitative method in embry- 

 ology ought to be established by degrees by the investigation 

 of not too compHcated examples. This method is still very 

 incomplete ; its period of trial is not yet over. It is only 

 by CARRYING OUT NUMEROUS MEASUREMENTS that 

 serious progress is to be expected. 



It is possible to investigate the development of many objects, 

 in which only a few properties have to be taken into considera- 

 tion and from which it is possible to collect numerous figures 

 without serious practical difficulties. 



EXAMPLES : The development of a phyllom, a trichom ; 

 a Spirogyra and other similar plants. In the animal kingdom, 

 the development of the legs, antennae, wings, etc., of articulate 

 animals (Arachnids, Crustacea, Hemiptera, Orthoptera, etc.), 

 the shell of Bivalve MoUusca, etc. 



The development of such comparatively simple objects is 

 governed by the same mechanical laws as that of a rabbit, a 

 chicken or a frog, because the individual evolution consists, in 

 each and all, of the passage of one state of equilibrium to another. 



When we want to discover laws of nature the best method 

 is, of course, to proceed from simplicity to complexity. This 

 logical line has been followed and is still followed in those 

 sciences which investigate inorganic nature. The founders of 

 modern chemistry (LAVOISIER, PRIESTLEY) began with 

 the study of very simple substances, such as water, carbon 

 dioxide, sulphur dioxide, mercuric oxide, etc. If they had tried, 

 at the end of the eighteenth century, to discover the constitu- 

 tion of complicated silicates, indigo or alkaloids, their efforts 

 would have been fruitless. HAUY has drawn up the funda- 

 mental laws of crystallography from the study of beautiful, 

 regular crystals ; later on he attacked complicated and 



