Alston, who had to earn his livelihood by medical practice, gave much time to the administration of the Botanic Gardens under his charge, and the elaborate lists which he prepared showing the disposition of plants in the Gardens, witness to his interest in their cultivation. His predilection in systematic arrangement was Tournefortian, and on the promulgation by Linnaeus of his "sexual system" in 1736, no writer was more trenchant than Alston in opposition to it, and by this he became widely known. His criticism was directed against it, not as a method of arranging plants by readily recognised characters, but from the standpoint of denial of the existence of sex. By various experiments as well as by argument, Alston endeavoured to disprove the necessity of the stamens for the development of fertile seed, citing cases of seed-production where no application of the "dust" from the stamens was possible--thus early recognising conditions which puzzled botanists for many generations afterwards and until the explanation of apogamy was supplied. One is tempted to wonder whether if the Linnaean system had not received the appellation "sexual" it would have roused the same condemnation from him as it did. From his published work, notably the _Dissertation on Botany_ (1754) a translation of a portion of his earlier _Tirocinium Botanicum Edinburgense_ (1740), as also from some MS. of his lectures which still exist, we recognise the clearness and vigour of mind of Alston, and the precision of the man is made abundantly evident in the beautiful copper-plate writing in old script of his MS. Page after page is filled without blot or correction, and the whole systematised and arranged without flaw. Anatomical questions were dealt with by him in consonance with the knowledge of the time, mainly resting on Malpighi; but there is no rational treatment of physiological subjects, and this is the more surprising inasmuch as he was in intimate correspondence with Hales, and ought to have been acquainted with the fundamental experimental work of that physiologist. It may be that the fragments of record from which we have to judge are insufficient for correct appraisement, but on all the evidence we possess we must conclude that the two volumes of his _Materia Medica_ give us a picture of the direction of his teaching, and that Botany in the hands of its leading expositor in Edinburgh was at this period only a hand-maid to Medicine.