CHAP, iv.] Origin of Tissues. 317

pure historic ground ; for the majority of the men who have developed modern doctrines since 1840 are still alive, and it must be uncertain whether the account here attempted may not be impugned on some ground or other. Owing to the extraordinary diversity of opinion that exists among botanists even on the most general questions in the science, it is extremely difficult to ascertain what can be considered as a common possession, an unfortunate condition of things, from which no science perhaps suffers so much as botany.

The extent to which individual botanists have contributed to the advance of phytotomy during the period under consider- ation will appear of itself from the following narrative ; and if we speak almost exclusively of Germans, it is for the simple reason that Englishmen from Crew's time till now can scarcely be said to have added anything to our knowledge of phyto- tomy ; the Italians also, once so gloriously represented by Malpighi, scarcely come under consideration in the questions now to be dealt with, while^ French botanists, represented by Mirbel in the preceding period, though they have produced many works on phytotomy since his time, have had no impor- tant share in deciding the fundamental questions of modern science.

In the preceding period it was necessary to take into consideration the increasing improvement of the microscope, in order to understand the development of opinion on vegetable structure ; but it is scarcely needful to do so after 1840. Since that time good and serviceable instruments with strong magnifying powers and clear definition have been within the reach of every phytotomist ; and though improve- ments are still being constantly made, yet the microscopes that were in the hands of skilful observers between 1840 and 1860 were fully adequate to deciding the new questions proposed to them. The chief improvement effected in the microscope during this period was the fitting it with apparatus for the

polarisation of light, and for the more convenient measurement