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: These differences may be caused by timolol - induced bradycardia and a compensatory increase in end - diastolic volume .
### Output Text:
These differences may be caused by <span class="chemical">timolol</span> - induced bradycardia and a compensatory increase in end - diastolic volume . 