In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: By any single behavioral criterion or a combination of these criteria , marked differences in response to toxic caffeine doses were observed between strains .
### Output Text:
By any single behavioral criterion or a combination of these criteria , marked differences in response to toxic <span class="chemical">caffeine</span> doses were observed between strains . 