374 A SHORT HISTORY OF SCIENCE 



PROGRESS IN BOTANY. The Linnsean system of classification 

 for the higher plants was a purely empirical system, based largely 

 upon the number of stamens and not involving any ideas of rela- 

 tionship through descent. B. de Jussieu (1699-1767), a friend and 

 pupil of Linnaeus, had proposed a different system of classifica- 

 tion in which weight is given to the totality of resemblances of 

 whatever kind, and this, which almost inevitably led to the close 

 association of related forms, was an important step toward a 

 natural classification, i.e. one based avowedly upon relationship, 

 common ancestry, or descent. De Jussieu's nephew Antoine de 

 Jussieu continued and extended his uncle's work. A. de Candolle 

 (1778-1841) later adopted and extended de Jussieu's system 

 which, with his own, now forms the basis of our present natural 

 system. It is noteworthy that this change of opinion in regard 

 to the relationship of the species of plants was ultimately effected 

 without theological protest. 1 



The discovery by Goethe of the homologies of the parts, and 

 by Linnaeus of the organs of sex, of the flower, were important 

 steps toward the modern theory of the evolution of plant life. 



PROGRESS IN MICROSCOPY. THE ACHROMATIC OBJECTIVE. 

 Compound microscopes, i.e. microscopes consisting of two lenses, 

 an objective and an eyepiece, were probably invented at about 

 the same time as telescopes, which likewise consist of two lenses 

 or systems of lenses. But because of their imperfections, in 

 respect especially to spherical and chromatic aberration, such 

 microscopes were often inferior for use to the best simple micro- 

 scopes. It was not until about 1835 that the compound micro- 

 scope, though invented in the seventeenth century, became the 

 superior instrument that it is to-day, through the accumulated 

 improvements of a number of workers, especially Amici, Lilly, 

 Lister, and Chevalier resulting in the achromatic objective, 

 free from both spherical and chromatic aberration. 



Almost immediately thereafter, with the new microscopy, 

 began a rich harvest of discoveries, in what Pasteur has called the 



1 For an account of Linnseua' attitude to the doctrine of the fixity of species see 

 A. D. White, Warfare of Science, Vol. I, pp. 47, 59, 60. 



