Early nineteenth-century zoology in Britain has been characterized as determined by the ideological concerns of its proponents. Taking the zoologist Robert E. Grant as an exemplary figure in this regard, this article offers a differently nuanced account of the conditions under which natural-philosophical knowledge concerning animal life was established in post-Napoleonic Britain. Whilst acknowledging the ideological import of concepts such as force and law, it points to an additional set of concerns amongst natural philosophers – that of appropriate tool use in investigation. Grant's studies in his native Edinburgh relied heavily on the use of microscopes. On his arrival in London, however, he entered a culture in which a different set of objects – museum specimens – held greater persuasive power. This article relates changes in Grant's ideas and practices to the uneven emphases on microscopic and museological evidence amongst European, Scottish and English natural philosophers at this time. In so doing, it identifies the reliance of London-based natural philosophers on museology as constituting a limiting effect on the kinds of claim that Grant sought to make regarding the nature of life.