THE PRIMORDIA 63 



laxge amount of variation within the limits of each species and 

 even of each " variety," and, moreover, numerous specific 

 differences which have never been mentioned and probably 

 never surmised. The differences between the genera Carabus 

 and Calosoma are described in the classical books in a few 

 words ; they are, in reality, very numerous. 



According to the HaeckeUan School, embryos in an early 

 state of development are to be compared to lower animals 

 and plants. We know that the latter are variable just as the 

 higher forms of Hfe. For instance, a number of algcB and 

 fungi are exceedingly plastic, even in properties upon which 

 classification has been based. 



We may ask the question, Why should embryos be less vari- 

 able than adult specimens or lower animals and plants ? It is 

 difficult to find any serious reason why embryos should be, as it 

 were, sheltered from the influence of plasticity. 



In certain groups, such as Mammalia and Birds, the condi- 

 tions of existence of the embryos ^ seem to be little variable, 

 and therefore it may be expected that plasticity is rather small. 

 Even here, however, the influence of complexity exists. It may 

 be remarked that an important part of our knowledge of the 

 embryology of mammalia and birds has been collected by 

 the investigation of such animals as the dog, the rabbit and the 

 chicken, three species which include numerous subspecies (so- 

 caUed races), innumerable specimens being hybrids of very 

 complex origin. In the books and memoirs in which the em- 

 bryology of these species has been described again and again 

 one finds hardly ever any information about the subspecies to 

 which the described specimens belong. Variation produced by 

 hybridization has been simply ignored. 



It often happens that two scholars, endowed with an equal 

 talent for observation and an equal mastery of technique, do 

 not agree with one another, and that a third embryologist, in- 

 vestigating the same subject, causes still more confusion by 

 pubHshing new discoveries. It may be supposed that com- 

 plexity is here at play and that different subspecies (and 

 hybrids) have been confounded under the name dog, or rabbit, 

 or chicken. 



The application of the quantitative method to embryology would 

 enable us to reduce the complicated notion of DEVELOPMENT 

 to terms of the simple notion of GROWTH of the primordia. It 

 would enable us to investigate the influence of external con- 

 ditions upon the development (growth) of one or several given 

 primordia. Since the simple properties are, on the whole, in- 

 dependent of each other, it may be anticipated that the line (curve) 

 of growth of each primordium may be modified separately 

 ^ For instance, temperature, light, food, etc. 



