1. INTRODUCTION
2. OBSERVATIONS
2.1. Sample Selection and Observations
2.2. Data Reduction
3. MAGNETIC RADIATIVE TRANSFER MODELING
4. RESULTS
4.1. Measured Class I Stellar Parameters
4.2. Magnetic Field of Class I and FS Sources
5. DISCUSSION
5.2. Correlation of Stellar Parameters with Magnetic Field Strength
5.3. Photospheric and Emission Lines in Our Sample of Class I and FS Sources
5.4. The Circumstellar Environment of Class I and FS Sources
6. CONCLUSION