10 LECTURE I. 



immediately after it, most probably in a change of the proper generative substances. 

 This is not the place to go into these matters more in detail ; we ought rather to 

 emphasise only that the theory of descent has been developed from the consideration 

 of variation in reproduction, from the fact of the accumulation of new peculiarities 

 in the varieties. These facts, in combination with the observation that from the 

 simplest forms of plants up to the most highly organised there is an unbroken series 

 of transitional or intermediate forms, have led to the hypothesis, as daring as 

 fruitful, that the most highly developed organisms have been gradually developed from 

 the simplest, by means of the progressive formation of varieties. This is essentially 

 the meaning of the theory of descent, which has during the last twenty years pro- 

 duced so great a revolution in the views of naturalists. We also cannot dispense with it 

 in our organographical considerations, simply for the reason that the understanding of 

 any organic form whatever is only possible if one looks upon it as the result of the 

 ever progressing organising tendency of the substance of the plant, i. e. of the 

 variation of other preceding organic forms ; or in other words, every organic form 

 is the outcome of a history, which is as old as the organic world itself. This 

 principle asserts itself far less in the consideration of the specific functions of given 

 organs, than in the comparison of their external form.s, and the natural system is the 

 attempt to state clearly this historical relationship of all plants to one another. Like 

 all new and comprehensive hypotheses, the theory of descent has also caused con- 

 fusion and devastation in weak minds, and perhaps done as much harm as good. 

 We shall be on our guard against laying any value whatever upon the excrescences 

 of the theory of descent, without casting aside the good and helpful which is 

 contained in its scientific nucleus. Even the above statements concerning typical, 

 rudimentary, reduced and metamorphosed forms, have a clear, actual meaning only 

 on the basis of the theory of descent, and so I shall, in our further considerations, 

 have recourse to the theory where the facts require it. 



Concerning one point I should wish to anticipate : viz. the use of the word 

 Purpose, a word which many fanatics of the theory of descent would, if possible, 

 banish entirely from the language. But the fact that, formerly, Purpose in the 

 mechanism of organisms was referred to causes other than now, is no reason for 

 robbing our language of a pregnant term. By the expression. This or that mechanism 

 has a Purpose in an organism, one understands really nothing more than that this 

 contributes to the ability of the organism to exist. It is now obvious, however, without 

 further discussion, that all properties of any organism must necessarily be so arranged, 

 that they at least do not call in question its existence under the circumstances of 

 life natural to it. ' To the purpose' means, therefore, in general, the same as ' capable 

 of existence,' and it would be foolishness to waste even a word as to whether one 

 may use the term in this sense or not. This implies, however, that there is 

 absolutely no scientific merit in maintaining of any organic mechanism whatever, 

 that it is in general to the purpose, or contributes to the capability of existence ; 

 since that is self-evident. On the other hand, it is in certain circumstances very 

 important and profitable to demonstrate how far, and under what conditions, a given 

 mechanism in the organism is of purpose ; in what way this contributes, in combination 

 with other mechanisms, to the capability of existence of a given organism ; and strictly 

 speaking, the whole of physiology is essentially occupied with such demonstrations. 



