The fact that there is no set discussion on the principle of divergence in the 1844 Essay, makes it clear why the joint paper read before the Linnean Society on July 1, 1858, included a letter{28} to Asa Gray, as well as an extract{29} from the Essay of 1844. It is clearly because the letter to Gray includes a discussion on divergence, and was thus, probably, the only document, including this subject, which could be appropriately made use of. It shows once more how great was the importance attached by its author to the principle of divergence. {28} The passage is given in the _Life and Letters_, ii. p. 124. {29} The extract consists of the section on _Natural Means of Selection_, p. 87. I have spoken of the hurried and condensed manner in which the sketch of 1842 is written; the style of the later Essay (1844) is more finished. It has, however, the air of an uncorrected MS. rather than of a book which has gone through the ordeal of proof sheets. It has not all the force and conciseness of the _Origin_, but it has a certain freshness which gives it a character of its own. It must be remembered that the _Origin_ was an abstract or condensation of a much bigger book, whereas the Essay of 1844 was an expansion of the sketch of 1842. It is not therefore surprising that in the _Origin_ there is occasionally evident a chafing against the author's self-imposed limitation. Whereas in the 1844 Essay there is an air of freedom, as if the author were letting himself go, rather than applying the curb. This quality of freshness and the fact that some questions were more fully discussed in 1844 than in 1859, makes the earlier work good reading even to those who are familiar with the _Origin_. The writing of this Essay "during the summer of 1844," as stated in the Autobiography{30}, and "from memory," as Darwin says elsewhere{31}, was a remarkable achievement, and possibly renders more conceivable the still greater feat of the writing of the _Origin_ between July 1858 and September 1859. {30} _Life and Letters_, i. p. 84. {31} _Life and Letters_, ii. p. 18. It is an interesting subject for speculation: what influence on the world the Essay of 1844 would have exercised, had it been published in place of the Origin. The author evidently thought of its publication in its present state as an undesirable expedient, as appears clearly from the following extracts from the _Life and Letters_, vol. ii. pp. 16--18: _C. Darwin to Mrs Darwin._ DOWN, _July 5, 1844_. "... I have just finished my sketch of my species theory. If, as I believe, my theory in time be accepted even by one competent judge, it will be a considerable step in science.